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+;;; undo-tree.el --- Treat undo history as a tree  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2009-2013  Free Software Foundation, Inc
+
+;; Author: Toby Cubitt <toby-undo-tree@dr-qubit.org>
+;; Version: 0.6.5
+;; Keywords: convenience, files, undo, redo, history, tree
+;; URL: http://www.dr-qubit.org/emacs.php
+;; Repository: http://www.dr-qubit.org/git/undo-tree.git
+
+;; This file is part of Emacs.
+;;
+;; This file is free software: you can redistribute it and/or modify it under
+;; the terms of the GNU General Public License as published by the Free
+;; Software Foundation, either version 3 of the License, or (at your option)
+;; any later version.
+;;
+;; This program is distributed in the hope that it will be useful, but WITHOUT
+;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+;; FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+;; more details.
+;;
+;; You should have received a copy of the GNU General Public License along
+;; with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+
+;;; Commentary:
+;;
+;; Emacs has a powerful undo system. Unlike the standard undo/redo system in
+;; most software, it allows you to recover *any* past state of a buffer
+;; (whereas the standard undo/redo system can lose past states as soon as you
+;; redo). However, this power comes at a price: many people find Emacs' undo
+;; system confusing and difficult to use, spawning a number of packages that
+;; replace it with the less powerful but more intuitive undo/redo system.
+;;
+;; Both the loss of data with standard undo/redo, and the confusion of Emacs'
+;; undo, stem from trying to treat undo history as a linear sequence of
+;; changes. It's not. The `undo-tree-mode' provided by this package replaces
+;; Emacs' undo system with a system that treats undo history as what it is: a
+;; branching tree of changes. This simple idea allows the more intuitive
+;; behaviour of the standard undo/redo system to be combined with the power of
+;; never losing any history. An added side bonus is that undo history can in
+;; some cases be stored more efficiently, allowing more changes to accumulate
+;; before Emacs starts discarding history.
+;;
+;; The only downside to this more advanced yet simpler undo system is that it
+;; was inspired by Vim. But, after all, most successful religions steal the
+;; best ideas from their competitors!
+;;
+;;
+;; Installation
+;; ============
+;;
+;; This package has only been tested with Emacs versions 24 and CVS. It should
+;; work in Emacs versions 22 and 23 too, but will not work without
+;; modifications in earlier versions of Emacs.
+;;
+;; To install `undo-tree-mode', make sure this file is saved in a directory in
+;; your `load-path', and add the line:
+;;
+;;   (require 'undo-tree)
+;;
+;; to your .emacs file. Byte-compiling undo-tree.el is recommended (e.g. using
+;; "M-x byte-compile-file" from within emacs).
+;;
+;; If you want to replace the standard Emacs' undo system with the
+;; `undo-tree-mode' system in all buffers, you can enable it globally by
+;; adding:
+;;
+;;   (global-undo-tree-mode)
+;;
+;; to your .emacs file.
+;;
+;;
+;; Quick-Start
+;; ===========
+;;
+;; If you're the kind of person who likes to jump in the car and drive,
+;; without bothering to first figure out whether the button on the left dips
+;; the headlights or operates the ejector seat (after all, you'll soon figure
+;; it out when you push it), then here's the minimum you need to know:
+;;
+;; `undo-tree-mode' and `global-undo-tree-mode'
+;;   Enable undo-tree mode (either in the current buffer or globally).
+;;
+;; C-_  C-/  (`undo-tree-undo')
+;;   Undo changes.
+;;
+;; M-_  C-?  (`undo-tree-redo')
+;;   Redo changes.
+;;
+;; `undo-tree-switch-branch'
+;;   Switch undo-tree branch.
+;;   (What does this mean? Better press the button and see!)
+;;
+;; C-x u  (`undo-tree-visualize')
+;;   Visualize the undo tree.
+;;   (Better try pressing this button too!)
+;;
+;; C-x r u  (`undo-tree-save-state-to-register')
+;;   Save current buffer state to register.
+;;
+;; C-x r U  (`undo-tree-restore-state-from-register')
+;;   Restore buffer state from register.
+;;
+;;
+;;
+;; In the undo-tree visualizer:
+;;
+;; <up>  p  C-p  (`undo-tree-visualize-undo')
+;;   Undo changes.
+;;
+;; <down>  n  C-n  (`undo-tree-visualize-redo')
+;;   Redo changes.
+;;
+;; <left>  b  C-b  (`undo-tree-visualize-switch-branch-left')
+;;   Switch to previous undo-tree branch.
+;;
+;; <right>  f  C-f  (`undo-tree-visualize-switch-branch-right')
+;;   Switch to next undo-tree branch.
+;;
+;; C-<up>  M-{  (`undo-tree-visualize-undo-to-x')
+;;   Undo changes up to last branch point.
+;;
+;; C-<down>  M-}  (`undo-tree-visualize-redo-to-x')
+;;   Redo changes down to next branch point.
+;;
+;; <down>  n  C-n  (`undo-tree-visualize-redo')
+;;   Redo changes.
+;;
+;; <mouse-1>  (`undo-tree-visualizer-mouse-set')
+;;   Set state to node at mouse click.
+;;
+;; t  (`undo-tree-visualizer-toggle-timestamps')
+;;   Toggle display of time-stamps.
+;;
+;; d  (`undo-tree-visualizer-toggle-diff')
+;;   Toggle diff display.
+;;
+;; s  (`undo-tree-visualizer-selection-mode')
+;;   Toggle keyboard selection mode.
+;;
+;; q  (`undo-tree-visualizer-quit')
+;;   Quit undo-tree-visualizer.
+;;
+;; C-q  (`undo-tree-visualizer-abort')
+;;   Abort undo-tree-visualizer.
+;;
+;; ,  <
+;;   Scroll left.
+;;
+;; .  >
+;;   Scroll right.
+;;
+;; <pgup>  M-v
+;;   Scroll up.
+;;
+;; <pgdown>  C-v
+;;   Scroll down.
+;;
+;;
+;;
+;; In visualizer selection mode:
+;;
+;; <up>  p  C-p  (`undo-tree-visualizer-select-previous')
+;;   Select previous node.
+;;
+;; <down>  n  C-n  (`undo-tree-visualizer-select-next')
+;;   Select next node.
+;;
+;; <left>  b  C-b  (`undo-tree-visualizer-select-left')
+;;   Select left sibling node.
+;;
+;; <right>  f  C-f  (`undo-tree-visualizer-select-right')
+;;   Select right sibling node.
+;;
+;; <pgup>  M-v
+;;   Select node 10 above.
+;;
+;; <pgdown>  C-v
+;;   Select node 10 below.
+;;
+;; <enter>  (`undo-tree-visualizer-set')
+;;   Set state to selected node and exit selection mode.
+;;
+;; s  (`undo-tree-visualizer-mode')
+;;   Exit selection mode.
+;;
+;; t  (`undo-tree-visualizer-toggle-timestamps')
+;;   Toggle display of time-stamps.
+;;
+;; d  (`undo-tree-visualizer-toggle-diff')
+;;   Toggle diff display.
+;;
+;; q  (`undo-tree-visualizer-quit')
+;;   Quit undo-tree-visualizer.
+;;
+;; C-q  (`undo-tree-visualizer-abort')
+;;   Abort undo-tree-visualizer.
+;;
+;; ,  <
+;;   Scroll left.
+;;
+;; .  >
+;;   Scroll right.
+;;
+;;
+;;
+;; Persistent undo history:
+;;
+;; Note: Requires Emacs version 24.3 or higher.
+;;
+;; `undo-tree-auto-save-history' (variable)
+;;    automatically save and restore undo-tree history along with buffer
+;;    (disabled by default)
+;;
+;; `undo-tree-save-history' (command)
+;;    manually save undo history to file
+;;
+;; `undo-tree-load-history' (command)
+;;    manually load undo history from file
+;;
+;;
+;;
+;; Compressing undo history:
+;;
+;;   Undo history files cannot grow beyond the maximum undo tree size, which
+;;   is limited by `undo-limit', `undo-strong-limit' and
+;;   `undo-outer-limit'. Nevertheless, undo history files can grow quite
+;;   large. If you want to automatically compress undo history, add the
+;;   following advice to your .emacs file (replacing ".gz" with the filename
+;;   extension of your favourite compression algorithm):
+;;
+;;   (defadvice undo-tree-make-history-save-file-name
+;;     (after undo-tree activate)
+;;     (setq ad-return-value (concat ad-return-value ".gz")))
+;;
+;;
+;;
+;;
+;; Undo Systems
+;; ============
+;;
+;; To understand the different undo systems, it's easiest to consider an
+;; example. Imagine you make a few edits in a buffer. As you edit, you
+;; accumulate a history of changes, which we might visualize as a string of
+;; past buffer states, growing downwards:
+;;
+;;                                o  (initial buffer state)
+;;                                |
+;;                                |
+;;                                o  (first edit)
+;;                                |
+;;                                |
+;;                                o  (second edit)
+;;                                |
+;;                                |
+;;                                x  (current buffer state)
+;;
+;;
+;; Now imagine that you undo the last two changes. We can visualize this as
+;; rewinding the current state back two steps:
+;;
+;;                                o  (initial buffer state)
+;;                                |
+;;                                |
+;;                                x  (current buffer state)
+;;                                |
+;;                                |
+;;                                o
+;;                                |
+;;                                |
+;;                                o
+;;
+;;
+;; However, this isn't a good representation of what Emacs' undo system
+;; does. Instead, it treats the undos as *new* changes to the buffer, and adds
+;; them to the history:
+;;
+;;                                o  (initial buffer state)
+;;                                |
+;;                                |
+;;                                o  (first edit)
+;;                                |
+;;                                |
+;;                                o  (second edit)
+;;                                |
+;;                                |
+;;                                x  (buffer state before undo)
+;;                                |
+;;                                |
+;;                                o  (first undo)
+;;                                |
+;;                                |
+;;                                x  (second undo)
+;;
+;;
+;; Actually, since the buffer returns to a previous state after an undo,
+;; perhaps a better way to visualize it is to imagine the string of changes
+;; turning back on itself:
+;;
+;;        (initial buffer state)  o
+;;                                |
+;;                                |
+;;                  (first edit)  o  x  (second undo)
+;;                                |  |
+;;                                |  |
+;;                 (second edit)  o  o  (first undo)
+;;                                | /
+;;                                |/
+;;                                o  (buffer state before undo)
+;;
+;; Treating undos as new changes might seem a strange thing to do. But the
+;; advantage becomes clear as soon as we imagine what happens when you edit
+;; the buffer again. Since you've undone a couple of changes, new edits will
+;; branch off from the buffer state that you've rewound to. Conceptually, it
+;; looks like this:
+;;
+;;                                o  (initial buffer state)
+;;                                |
+;;                                |
+;;                                o
+;;                                |\
+;;                                | \
+;;                                o  x  (new edit)
+;;                                |
+;;                                |
+;;                                o
+;;
+;; The standard undo/redo system only lets you go backwards and forwards
+;; linearly. So as soon as you make that new edit, it discards the old
+;; branch. Emacs' undo just keeps adding changes to the end of the string. So
+;; the undo history in the two systems now looks like this:
+;;
+;;            Undo/Redo:                      Emacs' undo
+;;
+;;               o                                o
+;;               |                                |
+;;               |                                |
+;;               o                                o  o
+;;               .\                               |  |\
+;;               . \                              |  | \
+;;               .  x  (new edit)                 o  o  |
+;;   (discarded  .                                | /   |
+;;     branch)   .                                |/    |
+;;               .                                o     |
+;;                                                      |
+;;                                                      |
+;;                                                      x  (new edit)
+;;
+;; Now, what if you change your mind about those undos, and decide you did
+;; like those other changes you'd made after all? With the standard undo/redo
+;; system, you're lost. There's no way to recover them, because that branch
+;; was discarded when you made the new edit.
+;;
+;; However, in Emacs' undo system, those old buffer states are still there in
+;; the undo history. You just have to rewind back through the new edit, and
+;; back through the changes made by the undos, until you reach them. Of
+;; course, since Emacs treats undos (even undos of undos!) as new changes,
+;; you're really weaving backwards and forwards through the history, all the
+;; time adding new changes to the end of the string as you go:
+;;
+;;                       o
+;;                       |
+;;                       |
+;;                       o  o     o  (undo new edit)
+;;                       |  |\    |\
+;;                       |  | \   | \
+;;                       o  o  |  |  o  (undo the undo)
+;;                       | /   |  |  |
+;;                       |/    |  |  |
+;;      (trying to get   o     |  |  x  (undo the undo)
+;;       to this state)        | /
+;;                             |/
+;;                             o
+;;
+;; So far, this is still reasonably intuitive to use. It doesn't behave so
+;; differently to standard undo/redo, except that by going back far enough you
+;; can access changes that would be lost in standard undo/redo.
+;;
+;; However, imagine that after undoing as just described, you decide you
+;; actually want to rewind right back to the initial state. If you're lucky,
+;; and haven't invoked any command since the last undo, you can just keep on
+;; undoing until you get back to the start:
+;;
+;;      (trying to get   o              x  (got there!)
+;;       to this state)  |              |
+;;                       |              |
+;;                       o  o     o     o  (keep undoing)
+;;                       |  |\    |\    |
+;;                       |  | \   | \   |
+;;                       o  o  |  |  o  o  (keep undoing)
+;;                       | /   |  |  | /
+;;                       |/    |  |  |/
+;;      (already undid   o     |  |  o  (got this far)
+;;       to this state)        | /
+;;                             |/
+;;                             o
+;;
+;; But if you're unlucky, and you happen to have moved the point (say) after
+;; getting to the state labelled "got this far", then you've "broken the undo
+;; chain". Hold on to something solid, because things are about to get
+;; hairy. If you try to undo now, Emacs thinks you're trying to undo the
+;; undos! So to get back to the initial state you now have to rewind through
+;; *all* the changes, including the undos you just did:
+;;
+;;      (trying to get   o                          x  (finally got there!)
+;;       to this state)  |                          |
+;;                       |                          |
+;;                       o  o     o     o     o     o
+;;                       |  |\    |\    |\    |\    |
+;;                       |  | \   | \   | \   | \   |
+;;                       o  o  |  |  o  o  o  |  o  o
+;;                       | /   |  |  | /   |  |  | /
+;;                       |/    |  |  |/    |  |  |/
+;;      (already undid   o     |  |  o<.   |  |  o
+;;       to this state)        | /     :   | /
+;;                             |/      :   |/
+;;                             o       :   o
+;;                                     :
+;;                             (got this far, but
+;;                              broke the undo chain)
+;;
+;; Confused?
+;;
+;; In practice you can just hold down the undo key until you reach the buffer
+;; state that you want. But whatever you do, don't move around in the buffer
+;; to *check* that you've got back to where you want! Because you'll break the
+;; undo chain, and then you'll have to traverse the entire string of undos
+;; again, just to get back to the point at which you broke the
+;; chain. Undo-in-region and commands such as `undo-only' help to make using
+;; Emacs' undo a little easier, but nonetheless it remains confusing for many
+;; people.
+;;
+;;
+;; So what does `undo-tree-mode' do? Remember the diagram we drew to represent
+;; the history we've been discussing (make a few edits, undo a couple of them,
+;; and edit again)? The diagram that conceptually represented our undo
+;; history, before we started discussing specific undo systems? It looked like
+;; this:
+;;
+;;                                o  (initial buffer state)
+;;                                |
+;;                                |
+;;                                o
+;;                                |\
+;;                                | \
+;;                                o  x  (current state)
+;;                                |
+;;                                |
+;;                                o
+;;
+;; Well, that's *exactly* what the undo history looks like to
+;; `undo-tree-mode'.  It doesn't discard the old branch (as standard undo/redo
+;; does), nor does it treat undos as new changes to be added to the end of a
+;; linear string of buffer states (as Emacs' undo does). It just keeps track
+;; of the tree of branching changes that make up the entire undo history.
+;;
+;; If you undo from this point, you'll rewind back up the tree to the previous
+;; state:
+;;
+;;                                o
+;;                                |
+;;                                |
+;;                                x  (undo)
+;;                                |\
+;;                                | \
+;;                                o  o
+;;                                |
+;;                                |
+;;                                o
+;;
+;; If you were to undo again, you'd rewind back to the initial state. If on
+;; the other hand you redo the change, you'll end up back at the bottom of the
+;; most recent branch:
+;;
+;;                                o  (undo takes you here)
+;;                                |
+;;                                |
+;;                                o  (start here)
+;;                                |\
+;;                                | \
+;;                                o  x  (redo takes you here)
+;;                                |
+;;                                |
+;;                                o
+;;
+;; So far, this is just like the standard undo/redo system. But what if you
+;; want to return to a buffer state located on a previous branch of the
+;; history? Since `undo-tree-mode' keeps the entire history, you simply need
+;; to tell it to switch to a different branch, and then redo the changes you
+;; want:
+;;
+;;                                o
+;;                                |
+;;                                |
+;;                                o  (start here, but switch
+;;                                |\  to the other branch)
+;;                                | \
+;;                        (redo)  o  o
+;;                                |
+;;                                |
+;;                        (redo)  x
+;;
+;; Now you're on the other branch, if you undo and redo changes you'll stay on
+;; that branch, moving up and down through the buffer states located on that
+;; branch. Until you decide to switch branches again, of course.
+;;
+;; Real undo trees might have multiple branches and sub-branches:
+;;
+;;                                o
+;;                            ____|______
+;;                           /           \
+;;                          o             o
+;;                      ____|__         __|
+;;                     /    |  \       /   \
+;;                    o     o   o     o     x
+;;                    |               |
+;;                   / \             / \
+;;                  o   o           o   o
+;;
+;; Trying to imagine what Emacs' undo would do as you move about such a tree
+;; will likely frazzle your brain circuits! But in `undo-tree-mode', you're
+;; just moving around this undo history tree. Most of the time, you'll
+;; probably only need to stay on the most recent branch, in which case it
+;; behaves like standard undo/redo, and is just as simple to understand. But
+;; if you ever need to recover a buffer state on a different branch, the
+;; possibility of switching between branches and accessing the full undo
+;; history is still there.
+;;
+;;
+;;
+;; The Undo-Tree Visualizer
+;; ========================
+;;
+;; Actually, it gets better. You don't have to imagine all these tree
+;; diagrams, because `undo-tree-mode' includes an undo-tree visualizer which
+;; draws them for you! In fact, it draws even better diagrams: it highlights
+;; the node representing the current buffer state, it highlights the current
+;; branch, and you can toggle the display of time-stamps (by hitting "t") and
+;; a diff of the undo changes (by hitting "d"). (There's one other tiny
+;; difference: the visualizer puts the most recent branch on the left rather
+;; than the right.)
+;;
+;; Bring up the undo tree visualizer whenever you want by hitting "C-x u".
+;;
+;; In the visualizer, the usual keys for moving up and down a buffer instead
+;; move up and down the undo history tree (e.g. the up and down arrow keys, or
+;; "C-n" and "C-p"). The state of the "parent" buffer (the buffer whose undo
+;; history you are visualizing) is updated as you move around the undo tree in
+;; the visualizer. If you reach a branch point in the visualizer, the usual
+;; keys for moving forward and backward in a buffer instead switch branch
+;; (e.g. the left and right arrow keys, or "C-f" and "C-b").
+;;
+;; Clicking with the mouse on any node in the visualizer will take you
+;; directly to that node, resetting the state of the parent buffer to the
+;; state represented by that node.
+;;
+;; You can also select nodes directly using the keyboard, by hitting "s" to
+;; toggle selection mode. The usual motion keys now allow you to move around
+;; the tree without changing the parent buffer. Hitting <enter> will reset the
+;; state of the parent buffer to the state represented by the currently
+;; selected node.
+;;
+;; It can be useful to see how long ago the parent buffer was in the state
+;; represented by a particular node in the visualizer. Hitting "t" in the
+;; visualizer toggles the display of time-stamps for all the nodes. (Note
+;; that, because of the way `undo-tree-mode' works, these time-stamps may be
+;; somewhat later than the true times, especially if it's been a long time
+;; since you last undid any changes.)
+;;
+;; To get some idea of what changes are represented by a given node in the
+;; tree, it can be useful to see a diff of the changes. Hit "d" in the
+;; visualizer to toggle a diff display. This normally displays a diff between
+;; the current state and the previous one, i.e. it shows you the changes that
+;; will be applied if you undo (move up the tree). However, the diff display
+;; really comes into its own in the visualizer's selection mode (see above),
+;; where it instead shows a diff between the current state and the currently
+;; selected state, i.e. it shows you the changes that will be applied if you
+;; reset to the selected state.
+;;
+;; (Note that the diff is generated by the Emacs `diff' command, and is
+;; displayed using `diff-mode'. See the corresponding customization groups if
+;; you want to customize the diff display.)
+;;
+;; Finally, hitting "q" will quit the visualizer, leaving the parent buffer in
+;; whatever state you ended at. Hitting "C-q" will abort the visualizer,
+;; returning the parent buffer to whatever state it was originally in when the
+;; visualizer was .
+;;
+;;
+;;
+;; Undo-in-Region
+;; ==============
+;;
+;; Emacs allows a very useful and powerful method of undoing only selected
+;; changes: when a region is active, only changes that affect the text within
+;; that region will be undone. With the standard Emacs undo system, changes
+;; produced by undoing-in-region naturally get added onto the end of the
+;; linear undo history:
+;;
+;;                       o
+;;                       |
+;;                       |  x  (second undo-in-region)
+;;                       o  |
+;;                       |  |
+;;                       |  o  (first undo-in-region)
+;;                       o  |
+;;                       | /
+;;                       |/
+;;                       o
+;;
+;; You can of course redo these undos-in-region as usual, by undoing the
+;; undos:
+;;
+;;                       o
+;;                       |
+;;                       |  o_
+;;                       o  | \
+;;                       |  |  |
+;;                       |  o  o  (undo the undo-in-region)
+;;                       o  |  |
+;;                       | /   |
+;;                       |/    |
+;;                       o     x  (undo the undo-in-region)
+;;
+;;
+;; In `undo-tree-mode', undo-in-region works similarly: when there's an active
+;; region, undoing only undoes changes that affect that region. However, the
+;; way these undos-in-region are recorded in the undo history is quite
+;; different. In `undo-tree-mode', undo-in-region creates a new branch in the
+;; undo history. The new branch consists of an undo step that undoes some of
+;; the changes that affect the current region, and another step that undoes
+;; the remaining changes needed to rejoin the previous undo history.
+;;
+;;      Previous undo history                Undo-in-region
+;;
+;;               o                                o
+;;               |                                |
+;;               |                                |
+;;               o                                o
+;;               |                                |\
+;;               |                                | \
+;;               o                                o  x  (undo-in-region)
+;;               |                                |  |
+;;               |                                |  |
+;;               x                                o  o
+;;
+;; As long as you don't change the active region after undoing-in-region,
+;; continuing to undo-in-region extends the new branch, pulling more changes
+;; that affect the current region into an undo step immediately above your
+;; current location in the undo tree, and pushing the point at which the new
+;; branch is attached further up the tree:
+;;
+;;      First undo-in-region                 Second undo-in-region
+;;
+;;               o                                o
+;;               |                                |\
+;;               |                                | \
+;;               o                                o  x  (undo-in-region)
+;;               |\                               |  |
+;;               | \                              |  |
+;;               o  x                             o  o
+;;               |  |                             |  |
+;;               |  |                             |  |
+;;               o  o                             o  o
+;;
+;; Redoing takes you back down the undo tree, as usual (as long as you haven't
+;; changed the active region after undoing-in-region, it doesn't matter if it
+;; is still active):
+;;
+;;                       o
+;;			 |\
+;;			 | \
+;;			 o  o
+;;			 |  |
+;;			 |  |
+;;			 o  o  (redo)
+;;			 |  |
+;;			 |  |
+;;			 o  x  (redo)
+;;
+;;
+;; What about redo-in-region? Obviously, this only makes sense if you have
+;; already undone some changes, so that there are some changes to redo!
+;; Redoing-in-region splits off a new branch of the undo history below your
+;; current location in the undo tree. This time, the new branch consists of a
+;; redo step that redoes some of the redo changes that affect the current
+;; region, followed by all the remaining redo changes.
+;;
+;;      Previous undo history                Redo-in-region
+;;
+;;               o                                o
+;;               |                                |
+;;               |                                |
+;;               x                                o
+;;               |                                |\
+;;               |                                | \
+;;               o                                o  x  (redo-in-region)
+;;               |                                |  |
+;;               |                                |  |
+;;               o                                o  o
+;;
+;; As long as you don't change the active region after redoing-in-region,
+;; continuing to redo-in-region extends the new branch, pulling more redo
+;; changes into a redo step immediately below your current location in the
+;; undo tree.
+;;
+;;      First redo-in-region                 Second redo-in-region
+;;
+;;          o                                     o
+;;          |                                     |
+;;          |                                     |
+;;          o                                     o
+;;          |\                                    |\
+;;          | \                                   | \
+;;          o  x  (redo-in-region)                o  o
+;;          |  |                                  |  |
+;;          |  |                                  |  |
+;;          o  o                                  o  x  (redo-in-region)
+;;                                                   |
+;;                                                   |
+;;                                                   o
+;;
+;; Note that undo-in-region and redo-in-region only ever add new changes to
+;; the undo tree, they *never* modify existing undo history. So you can always
+;; return to previous buffer states by switching to a previous branch of the
+;; tree.
+
+
+
+;;; Code:
+
+(eval-when-compile (require 'cl))
+(require 'diff)
+
+
+
+;;; =====================================================================
+;;;              Compatibility hacks for older Emacsen
+
+;; `characterp' isn't defined in Emacs versions < 23
+(unless (fboundp 'characterp)
+  (defalias 'characterp 'char-valid-p))
+
+;; `region-active-p' isn't defined in Emacs versions < 23
+(unless (fboundp 'region-active-p)
+  (defun region-active-p () (and transient-mark-mode mark-active)))
+
+
+;; `registerv' defstruct isn't defined in Emacs versions < 24
+(unless (fboundp 'registerv-make)
+  (defmacro registerv-make (data &rest _dummy) data))
+
+(unless (fboundp 'registerv-data)
+  (defmacro registerv-data (data) data))
+
+
+;; `diff-no-select' and `diff-file-local-copy' aren't defined in Emacs
+;; versions < 24 (copied and adapted from Emacs 24)
+(unless (fboundp 'diff-no-select)
+  (defun diff-no-select (old new &optional switches no-async buf)
+    ;; Noninteractive helper for creating and reverting diff buffers
+    (unless (bufferp new) (setq new (expand-file-name new)))
+    (unless (bufferp old) (setq old (expand-file-name old)))
+    (or switches (setq switches diff-switches)) ; If not specified, use default.
+    (unless (listp switches) (setq switches (list switches)))
+    (or buf (setq buf (get-buffer-create "*Diff*")))
+    (let* ((old-alt (diff-file-local-copy old))
+	   (new-alt (diff-file-local-copy new))
+	   (command
+	    (mapconcat 'identity
+		       `(,diff-command
+			 ;; Use explicitly specified switches
+			 ,@switches
+			 ,@(mapcar #'shell-quote-argument
+				   (nconc
+				    (when (or old-alt new-alt)
+				      (list "-L" (if (stringp old)
+						     old (prin1-to-string old))
+					    "-L" (if (stringp new)
+						     new (prin1-to-string new))))
+				    (list (or old-alt old)
+					  (or new-alt new)))))
+		       " "))
+	   (thisdir default-directory))
+      (with-current-buffer buf
+	(setq buffer-read-only t)
+	(buffer-disable-undo (current-buffer))
+	(let ((inhibit-read-only t))
+	  (erase-buffer))
+	(buffer-enable-undo (current-buffer))
+	(diff-mode)
+	(set (make-local-variable 'revert-buffer-function)
+	     (lambda (_ignore-auto _noconfirm)
+	       (diff-no-select old new switches no-async (current-buffer))))
+	(setq default-directory thisdir)
+	(let ((inhibit-read-only t))
+	  (insert command "\n"))
+	(if (and (not no-async) (fboundp 'start-process))
+	    (let ((proc (start-process "Diff" buf shell-file-name
+				       shell-command-switch command)))
+	      (set-process-filter proc 'diff-process-filter)
+	      (set-process-sentinel
+	       proc (lambda (proc _msg)
+		      (with-current-buffer (process-buffer proc)
+			(diff-sentinel (process-exit-status proc))
+			(if old-alt (delete-file old-alt))
+			(if new-alt (delete-file new-alt))))))
+	  ;; Async processes aren't available.
+	  (let ((inhibit-read-only t))
+	    (diff-sentinel
+	     (call-process shell-file-name nil buf nil
+			   shell-command-switch command))
+	    (if old-alt (delete-file old-alt))
+	    (if new-alt (delete-file new-alt)))))
+      buf)))
+
+(unless (fboundp 'diff-file-local-copy)
+  (defun diff-file-local-copy (file-or-buf)
+    (if (bufferp file-or-buf)
+	(with-current-buffer file-or-buf
+	  (let ((tempfile (make-temp-file "buffer-content-")))
+	    (write-region nil nil tempfile nil 'nomessage)
+	    tempfile))
+      (file-local-copy file-or-buf))))
+
+
+;; `user-error' isn't defined in Emacs < 24.3
+(unless (fboundp 'user-error)
+  (defalias 'user-error 'error)
+  ;; prevent debugger being called on user errors
+  (add-to-list 'debug-ignored-errors "^No further undo information")
+  (add-to-list 'debug-ignored-errors "^No further redo information")
+  (add-to-list 'debug-ignored-errors "^No further redo information for region"))
+
+
+
+
+
+;;; =====================================================================
+;;;              Global variables and customization options
+
+(defvar buffer-undo-tree nil
+  "Tree of undo entries in current buffer.")
+(put 'buffer-undo-tree 'permanent-local t)
+(make-variable-buffer-local 'buffer-undo-tree)
+
+
+(defgroup undo-tree nil
+  "Tree undo/redo."
+  :group 'undo)
+
+(defcustom undo-tree-mode-lighter " Undo-Tree"
+  "Lighter displayed in mode line
+when `undo-tree-mode' is enabled."
+  :group 'undo-tree
+  :type 'string)
+
+
+(defcustom undo-tree-incompatible-major-modes '(term-mode)
+  "List of major-modes in which `undo-tree-mode' should not be enabled.
+\(See `turn-on-undo-tree-mode'.\)"
+  :group 'undo-tree
+  :type '(repeat symbol))
+
+
+(defcustom undo-tree-enable-undo-in-region t
+  "When non-nil, enable undo-in-region.
+
+When undo-in-region is enabled, undoing or redoing when the
+region is active (in `transient-mark-mode') or with a prefix
+argument (not in `transient-mark-mode') only undoes changes
+within the current region."
+  :group 'undo-tree
+  :type 'boolean)
+
+
+(defcustom undo-tree-auto-save-history nil
+  "When non-nil, `undo-tree-mode' will save undo history to file
+when a buffer is saved to file.
+
+It will automatically load undo history when a buffer is loaded
+from file, if an undo save file exists.
+
+By default, undo-tree history is saved to a file called
+\".<buffer-file-name>.~undo-tree~\" in the same directory as the
+file itself. To save under a different directory, customize
+`undo-tree-history-directory-alist' (see the documentation for
+that variable for details).
+
+WARNING! `undo-tree-auto-save-history' will not work properly in
+Emacs versions prior to 24.3, so it cannot be enabled via
+the customization interface in versions earlier than that one. To
+ignore this warning and enable it regardless, set
+`undo-tree-auto-save-history' to a non-nil value outside of
+customize."
+  :group 'undo-tree
+  :type (if (version-list-< (version-to-list emacs-version) '(24 3))
+	    '(choice (const :tag "<disabled>" nil))
+	  'boolean))
+
+
+(defcustom undo-tree-history-directory-alist nil
+  "Alist of filename patterns and undo history directory names.
+Each element looks like (REGEXP . DIRECTORY).  Undo history for
+files with names matching REGEXP will be saved in DIRECTORY.
+DIRECTORY may be relative or absolute.  If it is absolute, so
+that all matching files are backed up into the same directory,
+the file names in this directory will be the full name of the
+file backed up with all directory separators changed to `!' to
+prevent clashes.  This will not work correctly if your filesystem
+truncates the resulting name.
+
+For the common case of all backups going into one directory, the
+alist should contain a single element pairing \".\" with the
+appropriate directory name.
+
+If this variable is nil, or it fails to match a filename, the
+backup is made in the original file's directory.
+
+On MS-DOS filesystems without long names this variable is always
+ignored."
+  :group 'undo-tree
+  :type '(repeat (cons (regexp :tag "Regexp matching filename")
+		       (directory :tag "Undo history directory name"))))
+
+
+
+(defcustom undo-tree-visualizer-relative-timestamps t
+  "When non-nil, display times relative to current time
+when displaying time stamps in visualizer.
+
+Otherwise, display absolute times."
+  :group 'undo-tree
+  :type 'boolean)
+
+
+(defcustom undo-tree-visualizer-timestamps nil
+  "When non-nil, display time-stamps by default
+in undo-tree visualizer.
+
+\\<undo-tree-visualizer-mode-map>You can always toggle time-stamps on and off \
+using \\[undo-tree-visualizer-toggle-timestamps], regardless of the
+setting of this variable."
+  :group 'undo-tree
+  :type 'boolean)
+
+
+(defcustom undo-tree-visualizer-diff nil
+  "When non-nil, display diff by default in undo-tree visualizer.
+
+\\<undo-tree-visualizer-mode-map>You can always toggle the diff display \
+using \\[undo-tree-visualizer-toggle-diff], regardless of the
+setting of this variable."
+  :group 'undo-tree
+  :type 'boolean)
+
+
+(defcustom undo-tree-visualizer-lazy-drawing 100
+  "When non-nil, use lazy undo-tree drawing in visualizer.
+
+Setting this to a number causes the visualizer to switch to lazy
+drawing when the number of nodes in the tree is larger than this
+value.
+
+Lazy drawing means that only the visible portion of the tree will
+be drawn initially, and the tree will be extended later as
+needed. For the most part, the only visible effect of this is to
+significantly speed up displaying the visualizer for very large
+trees.
+
+There is one potential negative effect of lazy drawing. Other
+branches of the tree will only be drawn once the node from which
+they branch off becomes visible. So it can happen that certain
+portions of the tree that would be shown with lazy drawing
+disabled, will not be drawn immediately when it is
+enabled. However, this effect is quite rare in practice."
+  :group 'undo-tree
+  :type '(choice (const :tag "never" nil)
+		 (const :tag "always" t)
+		 (integer :tag "> size")))
+
+
+(defface undo-tree-visualizer-default-face
+  '((((class color)) :foreground "gray"))
+  "Face used to draw undo-tree in visualizer."
+  :group 'undo-tree)
+
+(defface undo-tree-visualizer-current-face
+  '((((class color)) :foreground "red"))
+  "Face used to highlight current undo-tree node in visualizer."
+  :group 'undo-tree)
+
+(defface undo-tree-visualizer-active-branch-face
+  '((((class color) (background dark))
+     (:foreground "white" :weight bold))
+    (((class color) (background light))
+     (:foreground "black" :weight bold)))
+  "Face used to highlight active undo-tree branch in visualizer."
+  :group 'undo-tree)
+
+(defface undo-tree-visualizer-register-face
+  '((((class color)) :foreground "yellow"))
+  "Face used to highlight undo-tree nodes saved to a register
+in visualizer."
+  :group 'undo-tree)
+
+(defface undo-tree-visualizer-unmodified-face
+  '((((class color)) :foreground "cyan"))
+  "Face used to highlight nodes corresponding to unmodified buffers
+in visualizer."
+  :group 'undo-tree)
+
+
+(defvar undo-tree-visualizer-parent-buffer nil
+  "Parent buffer in visualizer.")
+(put 'undo-tree-visualizer-parent-buffer 'permanent-local t)
+(make-variable-buffer-local 'undo-tree-visualizer-parent-buffer)
+
+;; stores modification time of parent buffer's file, if any
+(defvar undo-tree-visualizer-parent-mtime nil)
+(put 'undo-tree-visualizer-parent-mtime 'permanent-local t)
+(make-variable-buffer-local 'undo-tree-visualizer-parent-mtime)
+
+;; stores current horizontal spacing needed for drawing undo-tree
+(defvar undo-tree-visualizer-spacing nil)
+(put 'undo-tree-visualizer-spacing 'permanent-local t)
+(make-variable-buffer-local 'undo-tree-visualizer-spacing)
+
+;; calculate horizontal spacing required for drawing tree with current
+;; settings
+(defsubst undo-tree-visualizer-calculate-spacing ()
+  (if undo-tree-visualizer-timestamps
+      (if undo-tree-visualizer-relative-timestamps 9 13)
+    3))
+
+;; holds node that was current when visualizer was invoked
+(defvar undo-tree-visualizer-initial-node nil)
+(put 'undo-tree-visualizer-initial-node 'permanent-local t)
+(make-variable-buffer-local 'undo-tree-visualizer-initial-node)
+
+;; holds currently selected node in visualizer selection mode
+(defvar undo-tree-visualizer-selected-node nil)
+(put 'undo-tree-visualizer-selected-node 'permanent-local t)
+(make-variable-buffer-local 'undo-tree-visualizer-selected)
+
+;; used to store nodes at edge of currently drawn portion of tree
+(defvar undo-tree-visualizer-needs-extending-down nil)
+(put 'undo-tree-visualizer-needs-extending-down 'permanent-local t)
+(make-variable-buffer-local 'undo-tree-visualizer-needs-extending-down)
+(defvar undo-tree-visualizer-needs-extending-up nil)
+(put 'undo-tree-visualizer-needs-extending-up 'permanent-local t)
+(make-variable-buffer-local 'undo-tree-visualizer-needs-extending-up)
+
+;; dynamically bound to t when undoing from visualizer, to inhibit
+;; `undo-tree-kill-visualizer' hook function in parent buffer
+(defvar undo-tree-inhibit-kill-visualizer nil)
+
+;; can be let-bound to a face name, used in drawing functions
+(defvar undo-tree-insert-face nil)
+
+;; visualizer buffer names
+(defconst undo-tree-visualizer-buffer-name " *undo-tree*")
+(defconst undo-tree-diff-buffer-name "*undo-tree Diff*")
+
+;; install history-auto-save hooks
+(add-hook 'write-file-functions 'undo-tree-save-history-hook)
+(add-hook 'find-file-hook 'undo-tree-load-history-hook)
+
+
+
+
+;;; =================================================================
+;;;                          Default keymaps
+
+(defvar undo-tree-map nil
+  "Keymap used in undo-tree-mode.")
+
+(unless undo-tree-map
+  (let ((map (make-sparse-keymap)))
+    ;; remap `undo' and `undo-only' to `undo-tree-undo'
+    (define-key map [remap undo] 'undo-tree-undo)
+    (define-key map [remap undo-only] 'undo-tree-undo)
+    ;; bind standard undo bindings (since these match redo counterparts)
+    (define-key map (kbd "C-/") 'undo-tree-undo)
+    (define-key map "\C-_" 'undo-tree-undo)
+    ;; redo doesn't exist normally, so define our own keybindings
+    (define-key map (kbd "C-?") 'undo-tree-redo)
+    (define-key map (kbd "M-_") 'undo-tree-redo)
+    ;; just in case something has defined `redo'...
+    (define-key map [remap redo] 'undo-tree-redo)
+    ;; we use "C-x u" for the undo-tree visualizer
+    (define-key map (kbd "\C-x u") 'undo-tree-visualize)
+    ;; bind register commands
+    (define-key map (kbd "C-x r u") 'undo-tree-save-state-to-register)
+    (define-key map (kbd "C-x r U") 'undo-tree-restore-state-from-register)
+    ;; set keymap
+    (setq undo-tree-map map)))
+
+
+(defvar undo-tree-visualizer-mode-map nil
+  "Keymap used in undo-tree visualizer.")
+
+(unless undo-tree-visualizer-mode-map
+  (let ((map (make-sparse-keymap)))
+    ;; vertical motion keys undo/redo
+    (define-key map [remap previous-line] 'undo-tree-visualize-undo)
+    (define-key map [remap next-line] 'undo-tree-visualize-redo)
+    (define-key map [up] 'undo-tree-visualize-undo)
+    (define-key map "p" 'undo-tree-visualize-undo)
+    (define-key map "\C-p" 'undo-tree-visualize-undo)
+    (define-key map [down] 'undo-tree-visualize-redo)
+    (define-key map "n" 'undo-tree-visualize-redo)
+    (define-key map "\C-n" 'undo-tree-visualize-redo)
+    ;; horizontal motion keys switch branch
+    (define-key map [remap forward-char]
+      'undo-tree-visualize-switch-branch-right)
+    (define-key map [remap backward-char]
+      'undo-tree-visualize-switch-branch-left)
+    (define-key map [right] 'undo-tree-visualize-switch-branch-right)
+    (define-key map "f" 'undo-tree-visualize-switch-branch-right)
+    (define-key map "\C-f" 'undo-tree-visualize-switch-branch-right)
+    (define-key map [left] 'undo-tree-visualize-switch-branch-left)
+    (define-key map "b" 'undo-tree-visualize-switch-branch-left)
+    (define-key map "\C-b" 'undo-tree-visualize-switch-branch-left)
+    ;; paragraph motion keys undo/redo to significant points in tree
+    (define-key map [remap backward-paragraph] 'undo-tree-visualize-undo-to-x)
+    (define-key map [remap forward-paragraph] 'undo-tree-visualize-redo-to-x)
+    (define-key map "\M-{" 'undo-tree-visualize-undo-to-x)
+    (define-key map "\M-}" 'undo-tree-visualize-redo-to-x)
+    (define-key map [C-up] 'undo-tree-visualize-undo-to-x)
+    (define-key map [C-down] 'undo-tree-visualize-redo-to-x)
+    ;; mouse sets buffer state to node at click
+    (define-key map [mouse-1] 'undo-tree-visualizer-mouse-set)
+    ;; toggle timestamps
+    (define-key map "t" 'undo-tree-visualizer-toggle-timestamps)
+    ;; toggle diff
+    (define-key map "d" 'undo-tree-visualizer-toggle-diff)
+    ;; toggle selection mode
+    (define-key map "s" 'undo-tree-visualizer-selection-mode)
+    ;; horizontal scrolling may be needed if the tree is very wide
+    (define-key map "," 'undo-tree-visualizer-scroll-left)
+    (define-key map "." 'undo-tree-visualizer-scroll-right)
+    (define-key map "<" 'undo-tree-visualizer-scroll-left)
+    (define-key map ">" 'undo-tree-visualizer-scroll-right)
+    ;; vertical scrolling may be needed if the tree is very tall
+    (define-key map [next] 'undo-tree-visualizer-scroll-up)
+    (define-key map [prior] 'undo-tree-visualizer-scroll-down)
+    ;; quit/abort visualizer
+    (define-key map "q" 'undo-tree-visualizer-quit)
+    (define-key map "\C-q" 'undo-tree-visualizer-abort)
+    ;; set keymap
+    (setq undo-tree-visualizer-mode-map map)))
+
+
+(defvar undo-tree-visualizer-selection-mode-map nil
+  "Keymap used in undo-tree visualizer selection mode.")
+
+(unless undo-tree-visualizer-selection-mode-map
+  (let ((map (make-sparse-keymap)))
+    ;; vertical motion keys move up and down tree
+    (define-key map [remap previous-line]
+      'undo-tree-visualizer-select-previous)
+    (define-key map [remap next-line]
+      'undo-tree-visualizer-select-next)
+    (define-key map [up] 'undo-tree-visualizer-select-previous)
+    (define-key map "p" 'undo-tree-visualizer-select-previous)
+    (define-key map "\C-p" 'undo-tree-visualizer-select-previous)
+    (define-key map [down] 'undo-tree-visualizer-select-next)
+    (define-key map "n" 'undo-tree-visualizer-select-next)
+    (define-key map "\C-n" 'undo-tree-visualizer-select-next)
+    ;; vertical scroll keys move up and down quickly
+    (define-key map [next]
+      (lambda () (interactive) (undo-tree-visualizer-select-next 10)))
+    (define-key map [prior]
+      (lambda () (interactive) (undo-tree-visualizer-select-previous 10)))
+    ;; horizontal motion keys move to left and right siblings
+    (define-key map [remap forward-char] 'undo-tree-visualizer-select-right)
+    (define-key map [remap backward-char] 'undo-tree-visualizer-select-left)
+    (define-key map [right] 'undo-tree-visualizer-select-right)
+    (define-key map "f" 'undo-tree-visualizer-select-right)
+    (define-key map "\C-f" 'undo-tree-visualizer-select-right)
+    (define-key map [left] 'undo-tree-visualizer-select-left)
+    (define-key map "b" 'undo-tree-visualizer-select-left)
+    (define-key map "\C-b" 'undo-tree-visualizer-select-left)
+    ;; horizontal scroll keys move left or right quickly
+    (define-key map ","
+      (lambda () (interactive) (undo-tree-visualizer-select-left 10)))
+    (define-key map "."
+      (lambda () (interactive) (undo-tree-visualizer-select-right 10)))
+    (define-key map "<"
+      (lambda () (interactive) (undo-tree-visualizer-select-left 10)))
+    (define-key map ">"
+      (lambda () (interactive) (undo-tree-visualizer-select-right 10)))
+    ;; <enter> sets buffer state to node at point
+    (define-key map "\r" 'undo-tree-visualizer-set)
+    ;; mouse selects node at click
+    (define-key map [mouse-1] 'undo-tree-visualizer-mouse-select)
+    ;; toggle diff
+    (define-key map "d" 'undo-tree-visualizer-selection-toggle-diff)
+    ;; set keymap
+    (setq undo-tree-visualizer-selection-mode-map map)))
+
+
+(defvar undo-tree-old-undo-menu-item nil)
+
+(defun undo-tree-update-menu-bar ()
+  "Update `undo-tree-mode' Edit menu items."
+  (if undo-tree-mode
+      (progn
+	;; save old undo menu item, and install undo/redo menu items
+	(setq undo-tree-old-undo-menu-item
+	      (cdr (assq 'undo (lookup-key global-map [menu-bar edit]))))
+	(define-key (lookup-key global-map [menu-bar edit])
+	  [undo] '(menu-item "Undo" undo-tree-undo
+			     :enable (and undo-tree-mode
+					  (not buffer-read-only)
+					  (not (eq t buffer-undo-list))
+					  (undo-tree-node-previous
+					   (undo-tree-current buffer-undo-tree)))
+			     :help "Undo last operation"))
+	(define-key-after (lookup-key global-map [menu-bar edit])
+	  [redo] '(menu-item "Redo" undo-tree-redo
+			     :enable (and undo-tree-mode
+					  (not buffer-read-only)
+					  (not (eq t buffer-undo-list))
+					  (undo-tree-node-next
+					   (undo-tree-current buffer-undo-tree)))
+			     :help "Redo last operation")
+	  'undo))
+    ;; uninstall undo/redo menu items
+    (define-key (lookup-key global-map [menu-bar edit])
+      [undo] undo-tree-old-undo-menu-item)
+    (define-key (lookup-key global-map [menu-bar edit])
+      [redo] nil)))
+
+(add-hook 'menu-bar-update-hook 'undo-tree-update-menu-bar)
+
+
+
+
+
+;;; =====================================================================
+;;;                     Undo-tree data structure
+
+(defstruct
+  (undo-tree
+   :named
+   (:constructor nil)
+   (:constructor make-undo-tree
+                 (&aux
+                  (root (undo-tree-make-node nil nil))
+                  (current root)
+                  (size 0)
+		  (count 0)
+		  (object-pool (make-hash-table :test 'eq :weakness 'value))))
+   ;;(:copier nil)
+   )
+  root current size count object-pool)
+
+
+
+(defstruct
+  (undo-tree-node
+   (:type vector)   ; create unnamed struct
+   (:constructor nil)
+   (:constructor undo-tree-make-node
+                 (previous undo
+		  &optional redo
+                  &aux
+                  (timestamp (current-time))
+                  (branch 0)))
+   (:constructor undo-tree-make-node-backwards
+                 (next-node undo
+		  &optional redo
+                  &aux
+                  (next (list next-node))
+                  (timestamp (current-time))
+                  (branch 0)))
+   (:copier nil))
+  previous next undo redo timestamp branch meta-data)
+
+
+(defmacro undo-tree-node-p (n)
+  (let ((len (length (undo-tree-make-node nil nil))))
+    `(and (vectorp ,n) (= (length ,n) ,len))))
+
+
+
+(defstruct
+  (undo-tree-region-data
+   (:type vector)   ; create unnamed struct
+   (:constructor nil)
+   (:constructor undo-tree-make-region-data
+		 (&optional undo-beginning undo-end
+			     redo-beginning redo-end))
+   (:constructor undo-tree-make-undo-region-data
+		 (undo-beginning undo-end))
+   (:constructor undo-tree-make-redo-region-data
+		 (redo-beginning redo-end))
+   (:copier nil))
+  undo-beginning undo-end redo-beginning redo-end)
+
+
+(defmacro undo-tree-region-data-p (r)
+  (let ((len (length (undo-tree-make-region-data))))
+    `(and (vectorp ,r) (= (length ,r) ,len))))
+
+(defmacro undo-tree-node-clear-region-data (node)
+  `(setf (undo-tree-node-meta-data ,node)
+	 (delq nil
+	       (delq :region
+		     (plist-put (undo-tree-node-meta-data ,node)
+				:region nil)))))
+
+
+(defmacro undo-tree-node-undo-beginning (node)
+  `(let ((r (plist-get (undo-tree-node-meta-data ,node) :region)))
+     (when (undo-tree-region-data-p r)
+       (undo-tree-region-data-undo-beginning r))))
+
+(defmacro undo-tree-node-undo-end (node)
+  `(let ((r (plist-get (undo-tree-node-meta-data ,node) :region)))
+     (when (undo-tree-region-data-p r)
+       (undo-tree-region-data-undo-end r))))
+
+(defmacro undo-tree-node-redo-beginning (node)
+  `(let ((r (plist-get (undo-tree-node-meta-data ,node) :region)))
+     (when (undo-tree-region-data-p r)
+       (undo-tree-region-data-redo-beginning r))))
+
+(defmacro undo-tree-node-redo-end (node)
+  `(let ((r (plist-get (undo-tree-node-meta-data ,node) :region)))
+     (when (undo-tree-region-data-p r)
+       (undo-tree-region-data-redo-end r))))
+
+
+(defsetf undo-tree-node-undo-beginning (node) (val)
+  `(let ((r (plist-get (undo-tree-node-meta-data ,node) :region)))
+     (unless (undo-tree-region-data-p r)
+       (setf (undo-tree-node-meta-data ,node)
+	     (plist-put (undo-tree-node-meta-data ,node) :region
+			(setq r (undo-tree-make-region-data)))))
+     (setf (undo-tree-region-data-undo-beginning r) ,val)))
+
+(defsetf undo-tree-node-undo-end (node) (val)
+  `(let ((r (plist-get (undo-tree-node-meta-data ,node) :region)))
+     (unless (undo-tree-region-data-p r)
+       (setf (undo-tree-node-meta-data ,node)
+	     (plist-put (undo-tree-node-meta-data ,node) :region
+			(setq r (undo-tree-make-region-data)))))
+     (setf (undo-tree-region-data-undo-end r) ,val)))
+
+(defsetf undo-tree-node-redo-beginning (node) (val)
+  `(let ((r (plist-get (undo-tree-node-meta-data ,node) :region)))
+     (unless (undo-tree-region-data-p r)
+       (setf (undo-tree-node-meta-data ,node)
+	     (plist-put (undo-tree-node-meta-data ,node) :region
+			(setq r (undo-tree-make-region-data)))))
+     (setf (undo-tree-region-data-redo-beginning r) ,val)))
+
+(defsetf undo-tree-node-redo-end (node) (val)
+  `(let ((r (plist-get (undo-tree-node-meta-data ,node) :region)))
+     (unless (undo-tree-region-data-p r)
+       (setf (undo-tree-node-meta-data ,node)
+	     (plist-put (undo-tree-node-meta-data ,node) :region
+			(setq r (undo-tree-make-region-data)))))
+     (setf (undo-tree-region-data-redo-end r) ,val)))
+
+
+
+(defstruct
+  (undo-tree-visualizer-data
+   (:type vector)   ; create unnamed struct
+   (:constructor nil)
+   (:constructor undo-tree-make-visualizer-data
+		 (&optional lwidth cwidth rwidth marker))
+   (:copier nil))
+  lwidth cwidth rwidth marker)
+
+
+(defmacro undo-tree-visualizer-data-p (v)
+  (let ((len (length (undo-tree-make-visualizer-data))))
+    `(and (vectorp ,v) (= (length ,v) ,len))))
+
+(defun undo-tree-node-clear-visualizer-data (node)
+  (let ((plist (undo-tree-node-meta-data node)))
+    (if (eq (car plist) :visualizer)
+	(setf (undo-tree-node-meta-data node) (nthcdr 2 plist))
+      (while (and plist (not (eq (cadr plist) :visualizer)))
+	(setq plist (cdr plist)))
+      (if plist (setcdr plist (nthcdr 3 plist))))))
+
+(defmacro undo-tree-node-lwidth (node)
+  `(let ((v (plist-get (undo-tree-node-meta-data ,node) :visualizer)))
+     (when (undo-tree-visualizer-data-p v)
+       (undo-tree-visualizer-data-lwidth v))))
+
+(defmacro undo-tree-node-cwidth (node)
+  `(let ((v (plist-get (undo-tree-node-meta-data ,node) :visualizer)))
+     (when (undo-tree-visualizer-data-p v)
+       (undo-tree-visualizer-data-cwidth v))))
+
+(defmacro undo-tree-node-rwidth (node)
+  `(let ((v (plist-get (undo-tree-node-meta-data ,node) :visualizer)))
+     (when (undo-tree-visualizer-data-p v)
+       (undo-tree-visualizer-data-rwidth v))))
+
+(defmacro undo-tree-node-marker (node)
+  `(let ((v (plist-get (undo-tree-node-meta-data ,node) :visualizer)))
+     (when (undo-tree-visualizer-data-p v)
+       (undo-tree-visualizer-data-marker v))))
+
+
+(defsetf undo-tree-node-lwidth (node) (val)
+  `(let ((v (plist-get (undo-tree-node-meta-data ,node) :visualizer)))
+     (unless (undo-tree-visualizer-data-p v)
+       (setf (undo-tree-node-meta-data ,node)
+	     (plist-put (undo-tree-node-meta-data ,node) :visualizer
+			(setq v (undo-tree-make-visualizer-data)))))
+     (setf (undo-tree-visualizer-data-lwidth v) ,val)))
+
+(defsetf undo-tree-node-cwidth (node) (val)
+  `(let ((v (plist-get (undo-tree-node-meta-data ,node) :visualizer)))
+     (unless (undo-tree-visualizer-data-p v)
+       (setf (undo-tree-node-meta-data ,node)
+	     (plist-put (undo-tree-node-meta-data ,node) :visualizer
+			(setq v (undo-tree-make-visualizer-data)))))
+     (setf (undo-tree-visualizer-data-cwidth v) ,val)))
+
+(defsetf undo-tree-node-rwidth (node) (val)
+  `(let ((v (plist-get (undo-tree-node-meta-data ,node) :visualizer)))
+     (unless (undo-tree-visualizer-data-p v)
+       (setf (undo-tree-node-meta-data ,node)
+	     (plist-put (undo-tree-node-meta-data ,node) :visualizer
+			(setq v (undo-tree-make-visualizer-data)))))
+     (setf (undo-tree-visualizer-data-rwidth v) ,val)))
+
+(defsetf undo-tree-node-marker (node) (val)
+  `(let ((v (plist-get (undo-tree-node-meta-data ,node) :visualizer)))
+     (unless (undo-tree-visualizer-data-p v)
+       (setf (undo-tree-node-meta-data ,node)
+	     (plist-put (undo-tree-node-meta-data ,node) :visualizer
+			(setq v (undo-tree-make-visualizer-data)))))
+     (setf (undo-tree-visualizer-data-marker v) ,val)))
+
+
+
+(defstruct
+  (undo-tree-register-data
+   (:type vector)
+   (:constructor nil)
+   (:constructor undo-tree-make-register-data (buffer node)))
+  buffer node)
+
+(defun undo-tree-register-data-p (data)
+  (and (vectorp data)
+       (= (length data) 2)
+       (undo-tree-node-p (undo-tree-register-data-node data))))
+
+(defun undo-tree-register-data-print-func (data)
+  (princ (format "an undo-tree state for buffer %s"
+		 (undo-tree-register-data-buffer data))))
+
+(defmacro undo-tree-node-register (node)
+  `(plist-get (undo-tree-node-meta-data ,node) :register))
+
+(defsetf undo-tree-node-register (node) (val)
+  `(setf (undo-tree-node-meta-data ,node)
+	 (plist-put (undo-tree-node-meta-data ,node) :register ,val)))
+
+
+
+
+;;; =====================================================================
+;;;              Basic undo-tree data structure functions
+
+(defun undo-tree-grow (undo)
+  "Add an UNDO node to current branch of `buffer-undo-tree'."
+  (let* ((current (undo-tree-current buffer-undo-tree))
+         (new (undo-tree-make-node current undo)))
+    (push new (undo-tree-node-next current))
+    (setf (undo-tree-current buffer-undo-tree) new)))
+
+
+(defun undo-tree-grow-backwards (node undo &optional redo)
+  "Add new node *above* undo-tree NODE, and return new node.
+Note that this will overwrite NODE's \"previous\" link, so should
+only be used on a detached NODE, never on nodes that are already
+part of `buffer-undo-tree'."
+  (let ((new (undo-tree-make-node-backwards node undo redo)))
+    (setf (undo-tree-node-previous node) new)
+    new))
+
+
+(defun undo-tree-splice-node (node splice)
+  "Splice NODE into undo tree, below node SPLICE.
+Note that this will overwrite NODE's \"next\" and \"previous\"
+links, so should only be used on a detached NODE, never on nodes
+that are already part of `buffer-undo-tree'."
+  (setf (undo-tree-node-next node) (undo-tree-node-next splice)
+	(undo-tree-node-branch node) (undo-tree-node-branch splice)
+	(undo-tree-node-previous node) splice
+	(undo-tree-node-next splice) (list node)
+	(undo-tree-node-branch splice) 0)
+  (dolist (n (undo-tree-node-next node))
+    (setf (undo-tree-node-previous n) node)))
+
+
+(defun undo-tree-snip-node (node)
+  "Snip NODE out of undo tree."
+  (let* ((parent (undo-tree-node-previous node))
+	 position p)
+    ;; if NODE is only child, replace parent's next links with NODE's
+    (if (= (length (undo-tree-node-next parent)) 0)
+	(setf (undo-tree-node-next parent) (undo-tree-node-next node)
+	      (undo-tree-node-branch parent) (undo-tree-node-branch node))
+      ;; otherwise...
+      (setq position (undo-tree-position node (undo-tree-node-next parent)))
+      (cond
+       ;; if active branch used do go via NODE, set parent's branch to active
+       ;; branch of NODE
+       ((= (undo-tree-node-branch parent) position)
+	(setf (undo-tree-node-branch parent)
+	      (+ position (undo-tree-node-branch node))))
+       ;; if active branch didn't go via NODE, update parent's branch to point
+       ;; to same node as before
+       ((> (undo-tree-node-branch parent) position)
+	(incf (undo-tree-node-branch parent)
+	      (1- (length (undo-tree-node-next node))))))
+      ;; replace NODE in parent's next list with NODE's entire next list
+      (if (= position 0)
+	  (setf (undo-tree-node-next parent)
+		(nconc (undo-tree-node-next node)
+		       (cdr (undo-tree-node-next parent))))
+	(setq p (nthcdr (1- position) (undo-tree-node-next parent)))
+	(setcdr p (nconc (undo-tree-node-next node) (cddr p)))))
+    ;; update previous links of NODE's children
+    (dolist (n (undo-tree-node-next node))
+      (setf (undo-tree-node-previous n) parent))))
+
+
+(defun undo-tree-mapc (--undo-tree-mapc-function-- node)
+  ;; Apply FUNCTION to NODE and to each node below it.
+  (let ((stack (list node))
+	n)
+    (while stack
+      (setq n (pop stack))
+      (funcall --undo-tree-mapc-function-- n)
+      (setq stack (append (undo-tree-node-next n) stack)))))
+
+
+(defmacro undo-tree-num-branches ()
+  "Return number of branches at current undo tree node."
+  '(length (undo-tree-node-next (undo-tree-current buffer-undo-tree))))
+
+
+(defun undo-tree-position (node list)
+  "Find the first occurrence of NODE in LIST.
+Return the index of the matching item, or nil of not found.
+Comparison is done with `eq'."
+  (let ((i 0))
+    (catch 'found
+      (while (progn
+               (when (eq node (car list)) (throw 'found i))
+               (incf i)
+               (setq list (cdr list))))
+      nil)))
+
+
+(defvar *undo-tree-id-counter* 0)
+(make-variable-buffer-local '*undo-tree-id-counter*)
+
+(defmacro undo-tree-generate-id ()
+  ;; Generate a new, unique id (uninterned symbol).
+  ;; The name is made by appending a number to "undo-tree-id".
+  ;; (Copied from CL package `gensym'.)
+  `(let ((num (prog1 *undo-tree-id-counter* (incf *undo-tree-id-counter*))))
+     (make-symbol (format "undo-tree-id%d" num))))
+
+
+(defun undo-tree-decircle (undo-tree)
+  ;; Nullify PREVIOUS links of UNDO-TREE nodes, to make UNDO-TREE data
+  ;; structure non-circular.
+  (undo-tree-mapc
+   (lambda (node)
+     (dolist (n (undo-tree-node-next node))
+       (setf (undo-tree-node-previous n) nil)))
+   (undo-tree-root undo-tree)))
+
+
+(defun undo-tree-recircle (undo-tree)
+  ;; Recreate PREVIOUS links of UNDO-TREE nodes, to restore circular UNDO-TREE
+  ;; data structure.
+  (undo-tree-mapc
+   (lambda (node)
+     (dolist (n (undo-tree-node-next node))
+       (setf (undo-tree-node-previous n) node)))
+   (undo-tree-root undo-tree)))
+
+
+
+
+;;; =====================================================================
+;;;             Undo list and undo changeset utility functions
+
+(defmacro undo-list-marker-elt-p (elt)
+  `(markerp (car-safe ,elt)))
+
+(defmacro undo-list-GCd-marker-elt-p (elt)
+  ;; Return t if ELT is a marker element whose marker has been moved to the
+  ;; object-pool, so may potentially have been garbage-collected.
+  ;; Note: Valid marker undo elements should be uniquely identified as cons
+  ;; cells with a symbol in the car (replacing the marker), and a number in
+  ;; the cdr. However, to guard against future changes to undo element
+  ;; formats, we perform an additional redundant check on the symbol name.
+  `(and (car-safe ,elt)
+	(symbolp (car ,elt))
+	(let ((str (symbol-name (car ,elt))))
+	  (and (> (length str) 12)
+	       (string= (substring str 0 12) "undo-tree-id")))
+	(numberp (cdr-safe ,elt))))
+
+
+(defun undo-tree-move-GC-elts-to-pool (elt)
+  ;; Move elements that can be garbage-collected into `buffer-undo-tree'
+  ;; object pool, substituting a unique id that can be used to retrieve them
+  ;; later. (Only markers require this treatment currently.)
+  (when (undo-list-marker-elt-p elt)
+    (let ((id (undo-tree-generate-id)))
+      (puthash id (car elt) (undo-tree-object-pool buffer-undo-tree))
+      (setcar elt id))))
+
+
+(defun undo-tree-restore-GC-elts-from-pool (elt)
+  ;; Replace object id's in ELT with corresponding objects from
+  ;; `buffer-undo-tree' object pool and return modified ELT, or return nil if
+  ;; any object in ELT has been garbage-collected.
+  (if (undo-list-GCd-marker-elt-p elt)
+      (when (setcar elt (gethash (car elt)
+				 (undo-tree-object-pool buffer-undo-tree)))
+	elt)
+    elt))
+
+
+(defun undo-list-clean-GCd-elts (undo-list)
+  ;; Remove object id's from UNDO-LIST that refer to elements that have been
+  ;; garbage-collected. UNDO-LIST is modified by side-effect.
+  (while (undo-list-GCd-marker-elt-p (car undo-list))
+    (unless (gethash (caar undo-list)
+		     (undo-tree-object-pool buffer-undo-tree))
+      (setq undo-list (cdr undo-list))))
+  (let ((p undo-list))
+    (while (cdr p)
+      (when (and (undo-list-GCd-marker-elt-p (cadr p))
+		 (null (gethash (car (cadr p))
+				(undo-tree-object-pool buffer-undo-tree))))
+	(setcdr p (cddr p)))
+      (setq p (cdr p))))
+  undo-list)
+
+
+(defun undo-list-pop-changeset (&optional discard-pos)
+  ;; Pop changeset from `buffer-undo-list'. If DISCARD-POS is non-nil, discard
+  ;; any position entries from changeset.
+
+  ;; discard undo boundaries and (if DISCARD-POS is non-nil) position entries
+  ;; at head of undo list
+  (while (or (null (car buffer-undo-list))
+	     (and discard-pos (integerp (car buffer-undo-list))))
+    (setq buffer-undo-list (cdr buffer-undo-list)))
+  ;; pop elements up to next undo boundary, discarding position entries if
+  ;; DISCARD-POS is non-nil
+  (if (eq (car buffer-undo-list) 'undo-tree-canary)
+      (push nil buffer-undo-list)
+    (let* ((changeset (list (pop buffer-undo-list)))
+           (p changeset))
+      (while (progn
+	       (undo-tree-move-GC-elts-to-pool (car p))
+	       (while (and discard-pos (integerp (car buffer-undo-list)))
+		 (setq buffer-undo-list (cdr buffer-undo-list)))
+	       (and (car buffer-undo-list)
+		    (not (eq (car buffer-undo-list) 'undo-tree-canary))))
+        (setcdr p (list (pop buffer-undo-list)))
+	(setq p (cdr p)))
+      changeset)))
+
+
+(defun undo-tree-copy-list (undo-list)
+  ;; Return a deep copy of first changeset in `undo-list'. Object id's are
+  ;; replaced by corresponding objects from `buffer-undo-tree' object-pool.
+  (when undo-list
+    (let (copy p)
+      ;; if first element contains an object id, replace it with object from
+      ;; pool, discarding element entirely if it's been GC'd
+      (while (null copy)
+	(setq copy
+	      (undo-tree-restore-GC-elts-from-pool (pop undo-list))))
+      (setq copy (list copy)
+	    p copy)
+      ;; copy remaining elements, replacing object id's with objects from
+      ;; pool, or discarding them entirely if they've been GC'd
+      (while undo-list
+	(when (setcdr p (undo-tree-restore-GC-elts-from-pool
+			 (undo-copy-list-1 (pop undo-list))))
+	  (setcdr p (list (cdr p)))
+	  (setq p (cdr p))))
+      copy)))
+
+
+
+(defun undo-list-transfer-to-tree ()
+  ;; Transfer entries accumulated in `buffer-undo-list' to `buffer-undo-tree'.
+
+  ;; `undo-list-transfer-to-tree' should never be called when undo is disabled
+  ;; (i.e. `buffer-undo-tree' is t)
+  (assert (not (eq buffer-undo-tree t)))
+
+  ;; if `buffer-undo-tree' is empty, create initial undo-tree
+  (when (null buffer-undo-tree) (setq buffer-undo-tree (make-undo-tree)))
+  ;; make sure there's a canary at end of `buffer-undo-list'
+  (when (null buffer-undo-list)
+    (setq buffer-undo-list '(nil undo-tree-canary)))
+
+  (unless (or (eq (cadr buffer-undo-list) 'undo-tree-canary)
+	      (eq (car buffer-undo-list) 'undo-tree-canary))
+    ;; create new node from first changeset in `buffer-undo-list', save old
+    ;; `buffer-undo-tree' current node, and make new node the current node
+    (let* ((node (undo-tree-make-node nil (undo-list-pop-changeset)))
+	   (splice (undo-tree-current buffer-undo-tree))
+	   (size (undo-list-byte-size (undo-tree-node-undo node)))
+	   (count 1))
+      (setf (undo-tree-current buffer-undo-tree) node)
+      ;; grow tree fragment backwards using `buffer-undo-list' changesets
+      (while (and buffer-undo-list
+		  (not (eq (cadr buffer-undo-list) 'undo-tree-canary)))
+	(setq node
+	      (undo-tree-grow-backwards node (undo-list-pop-changeset)))
+	(incf size (undo-list-byte-size (undo-tree-node-undo node)))
+	(incf count))
+      ;; if no undo history has been discarded from `buffer-undo-list' since
+      ;; last transfer, splice new tree fragment onto end of old
+      ;; `buffer-undo-tree' current node
+      (if (or (eq (cadr buffer-undo-list) 'undo-tree-canary)
+	      (eq (car buffer-undo-list) 'undo-tree-canary))
+	  (progn
+	    (setf (undo-tree-node-previous node) splice)
+	    (push node (undo-tree-node-next splice))
+	    (setf (undo-tree-node-branch splice) 0)
+	    (incf (undo-tree-size buffer-undo-tree) size)
+	    (incf (undo-tree-count buffer-undo-tree) count))
+	;; if undo history has been discarded, replace entire
+	;; `buffer-undo-tree' with new tree fragment
+	(setq node (undo-tree-grow-backwards node nil))
+	(setf (undo-tree-root buffer-undo-tree) node)
+	(setq buffer-undo-list '(nil undo-tree-canary))
+	(setf (undo-tree-size buffer-undo-tree) size)
+	(setf (undo-tree-count buffer-undo-tree) count)
+	(setq buffer-undo-list '(nil undo-tree-canary))))
+    ;; discard undo history if necessary
+    (undo-tree-discard-history)))
+
+
+(defun undo-list-byte-size (undo-list)
+  ;; Return size (in bytes) of UNDO-LIST
+  (let ((size 0) (p undo-list))
+    (while p
+      (incf size 8)  ; cons cells use up 8 bytes
+      (when (and (consp (car p)) (stringp (caar p)))
+        (incf size (string-bytes (caar p))))
+      (setq p (cdr p)))
+    size))
+
+
+
+(defun undo-list-rebuild-from-tree ()
+  "Rebuild `buffer-undo-list' from information in `buffer-undo-tree'."
+  (unless (eq buffer-undo-list t)
+    (undo-list-transfer-to-tree)
+    (setq buffer-undo-list nil)
+    (when buffer-undo-tree
+      (let ((stack (list (list (undo-tree-root buffer-undo-tree)))))
+	(push (sort (mapcar 'identity (undo-tree-node-next (caar stack)))
+		    (lambda (a b)
+		      (time-less-p (undo-tree-node-timestamp a)
+				   (undo-tree-node-timestamp b))))
+	      stack)
+	;; Traverse tree in depth-and-oldest-first order, but add undo records
+	;; on the way down, and redo records on the way up.
+	(while (or (car stack)
+		   (not (eq (car (nth 1 stack))
+			    (undo-tree-current buffer-undo-tree))))
+	  (if (car stack)
+	      (progn
+		(setq buffer-undo-list
+		      (append (undo-tree-node-undo (caar stack))
+			      buffer-undo-list))
+		(undo-boundary)
+		(push (sort (mapcar 'identity
+				    (undo-tree-node-next (caar stack)))
+			    (lambda (a b)
+			      (time-less-p (undo-tree-node-timestamp a)
+					   (undo-tree-node-timestamp b))))
+		      stack))
+	    (pop stack)
+	    (setq buffer-undo-list
+		  (append (undo-tree-node-redo (caar stack))
+			  buffer-undo-list))
+	    (undo-boundary)
+	    (pop (car stack))))))))
+
+
+
+
+;;; =====================================================================
+;;;                History discarding utility functions
+
+(defun undo-tree-oldest-leaf (node)
+  ;; Return oldest leaf node below NODE.
+  (while (undo-tree-node-next node)
+    (setq node
+          (car (sort (mapcar 'identity (undo-tree-node-next node))
+                     (lambda (a b)
+                       (time-less-p (undo-tree-node-timestamp a)
+                                    (undo-tree-node-timestamp b)))))))
+  node)
+
+
+(defun undo-tree-discard-node (node)
+  ;; Discard NODE from `buffer-undo-tree', and return next in line for
+  ;; discarding.
+
+  ;; don't discard current node
+  (unless (eq node (undo-tree-current buffer-undo-tree))
+
+    ;; discarding root node...
+    (if (eq node (undo-tree-root buffer-undo-tree))
+        (cond
+         ;; should always discard branches before root
+         ((> (length (undo-tree-node-next node)) 1)
+          (error "Trying to discard undo-tree root which still\
+ has multiple branches"))
+         ;; don't discard root if current node is only child
+         ((eq (car (undo-tree-node-next node))
+              (undo-tree-current buffer-undo-tree))
+	  nil)
+	 ;; discard root
+         (t
+	  ;; clear any register referring to root
+	  (let ((r (undo-tree-node-register node)))
+	    (when (and r (eq (get-register r) node))
+	      (set-register r nil)))
+          ;; make child of root into new root
+          (setq node (setf (undo-tree-root buffer-undo-tree)
+                           (car (undo-tree-node-next node))))
+	  ;; update undo-tree size
+	  (decf (undo-tree-size buffer-undo-tree)
+		(+ (undo-list-byte-size (undo-tree-node-undo node))
+		   (undo-list-byte-size (undo-tree-node-redo node))))
+	  (decf (undo-tree-count buffer-undo-tree))
+	  ;; discard new root's undo data and PREVIOUS link
+	  (setf (undo-tree-node-undo node) nil
+		(undo-tree-node-redo node) nil
+		(undo-tree-node-previous node) nil)
+          ;; if new root has branches, or new root is current node, next node
+          ;; to discard is oldest leaf, otherwise it's new root
+          (if (or (> (length (undo-tree-node-next node)) 1)
+                  (eq (car (undo-tree-node-next node))
+                      (undo-tree-current buffer-undo-tree)))
+              (undo-tree-oldest-leaf node)
+            node)))
+
+      ;; discarding leaf node...
+      (let* ((parent (undo-tree-node-previous node))
+             (current (nth (undo-tree-node-branch parent)
+                           (undo-tree-node-next parent))))
+	;; clear any register referring to the discarded node
+	(let ((r (undo-tree-node-register node)))
+	  (when (and r (eq (get-register r) node))
+	    (set-register r nil)))
+	;; update undo-tree size
+	(decf (undo-tree-size buffer-undo-tree)
+	      (+ (undo-list-byte-size (undo-tree-node-undo node))
+		 (undo-list-byte-size (undo-tree-node-redo node))))
+	(decf (undo-tree-count buffer-undo-tree))
+	;; discard leaf
+        (setf (undo-tree-node-next parent)
+                (delq node (undo-tree-node-next parent))
+              (undo-tree-node-branch parent)
+                (undo-tree-position current (undo-tree-node-next parent)))
+        ;; if parent has branches, or parent is current node, next node to
+        ;; discard is oldest leaf, otherwise it's the parent itself
+        (if (or (eq parent (undo-tree-current buffer-undo-tree))
+                (and (undo-tree-node-next parent)
+                     (or (not (eq parent (undo-tree-root buffer-undo-tree)))
+                         (> (length (undo-tree-node-next parent)) 1))))
+            (undo-tree-oldest-leaf parent)
+          parent)))))
+
+
+
+(defun undo-tree-discard-history ()
+  "Discard undo history until we're within memory usage limits
+set by `undo-limit', `undo-strong-limit' and `undo-outer-limit'."
+
+  (when (> (undo-tree-size buffer-undo-tree) undo-limit)
+    ;; if there are no branches off root, first node to discard is root;
+    ;; otherwise it's leaf node at botom of oldest branch
+    (let ((node (if (> (length (undo-tree-node-next
+                                (undo-tree-root buffer-undo-tree))) 1)
+                    (undo-tree-oldest-leaf (undo-tree-root buffer-undo-tree))
+                  (undo-tree-root buffer-undo-tree))))
+
+      ;; discard nodes until memory use is within `undo-strong-limit'
+      (while (and node
+                  (> (undo-tree-size buffer-undo-tree) undo-strong-limit))
+        (setq node (undo-tree-discard-node node)))
+
+      ;; discard nodes until next node to discard would bring memory use
+      ;; within `undo-limit'
+      (while (and node
+		  ;; check first if last discard has brought us within
+		  ;; `undo-limit', in case we can avoid more expensive
+		  ;; `undo-strong-limit' calculation
+		  ;; Note: this assumes undo-strong-limit > undo-limit;
+		  ;;       if not, effectively undo-strong-limit = undo-limit
+		  (> (undo-tree-size buffer-undo-tree) undo-limit)
+                  (> (- (undo-tree-size buffer-undo-tree)
+			;; if next node to discard is root, the memory we
+			;; free-up comes from discarding changesets from its
+			;; only child...
+			(if (eq node (undo-tree-root buffer-undo-tree))
+			    (+ (undo-list-byte-size
+				(undo-tree-node-undo
+				 (car (undo-tree-node-next node))))
+			       (undo-list-byte-size
+				(undo-tree-node-redo
+				 (car (undo-tree-node-next node)))))
+			  ;; ...otherwise, it comes from discarding changesets
+			  ;; from along with the node itself
+			  (+ (undo-list-byte-size (undo-tree-node-undo node))
+			     (undo-list-byte-size (undo-tree-node-redo node)))
+			  ))
+                     undo-limit))
+        (setq node (undo-tree-discard-node node)))
+
+      ;; if we're still over the `undo-outer-limit', discard entire history
+      (when (> (undo-tree-size buffer-undo-tree) undo-outer-limit)
+        ;; query first if `undo-ask-before-discard' is set
+        (if undo-ask-before-discard
+            (when (yes-or-no-p
+                   (format
+                    "Buffer `%s' undo info is %d bytes long;  discard it? "
+                    (buffer-name) (undo-tree-size buffer-undo-tree)))
+              (setq buffer-undo-tree nil))
+          ;; otherwise, discard and display warning
+          (display-warning
+           '(undo discard-info)
+           (concat
+            (format "Buffer `%s' undo info was %d bytes long.\n"
+                    (buffer-name) (undo-tree-size buffer-undo-tree))
+            "The undo info was discarded because it exceeded\
+ `undo-outer-limit'.
+
+This is normal if you executed a command that made a huge change
+to the buffer. In that case, to prevent similar problems in the
+future, set `undo-outer-limit' to a value that is large enough to
+cover the maximum size of normal changes you expect a single
+command to make, but not so large that it might exceed the
+maximum memory allotted to Emacs.
+
+If you did not execute any such command, the situation is
+probably due to a bug and you should report it.
+
+You can disable the popping up of this buffer by adding the entry
+\(undo discard-info) to the user option `warning-suppress-types',
+which is defined in the `warnings' library.\n")
+           :warning)
+          (setq buffer-undo-tree nil)))
+      )))
+
+
+
+
+;;; =====================================================================
+;;;                   Visualizer utility functions
+
+(defun undo-tree-compute-widths (node)
+  "Recursively compute widths for nodes below NODE."
+  (let ((stack (list node))
+        res)
+    (while stack
+      ;; try to compute widths for node at top of stack
+      (if (undo-tree-node-p
+           (setq res (undo-tree-node-compute-widths (car stack))))
+          ;; if computation fails, it returns a node whose widths still need
+          ;; computing, which we push onto the stack
+          (push res stack)
+        ;; otherwise, store widths and remove it from stack
+        (setf (undo-tree-node-lwidth (car stack)) (aref res 0)
+              (undo-tree-node-cwidth (car stack)) (aref res 1)
+              (undo-tree-node-rwidth (car stack)) (aref res 2))
+        (pop stack)))))
+
+
+(defun undo-tree-node-compute-widths (node)
+  ;; Compute NODE's left-, centre-, and right-subtree widths. Returns widths
+  ;; (in a vector) if successful. Otherwise, returns a node whose widths need
+  ;; calculating before NODE's can be calculated.
+  (let ((num-children (length (undo-tree-node-next node)))
+        (lwidth 0) (cwidth 0) (rwidth 0) p)
+    (catch 'need-widths
+      (cond
+       ;; leaf nodes have 0 width
+       ((= 0 num-children)
+        (setf cwidth 1
+              (undo-tree-node-lwidth node) 0
+              (undo-tree-node-cwidth node) 1
+              (undo-tree-node-rwidth node) 0))
+
+       ;; odd number of children
+       ((= (mod num-children 2) 1)
+        (setq p (undo-tree-node-next node))
+        ;; compute left-width
+        (dotimes (i (/ num-children 2))
+          (if (undo-tree-node-lwidth (car p))
+              (incf lwidth (+ (undo-tree-node-lwidth (car p))
+                              (undo-tree-node-cwidth (car p))
+                              (undo-tree-node-rwidth (car p))))
+            ;; if child's widths haven't been computed, return that child
+            (throw 'need-widths (car p)))
+          (setq p (cdr p)))
+        (if (undo-tree-node-lwidth (car p))
+            (incf lwidth (undo-tree-node-lwidth (car p)))
+          (throw 'need-widths (car p)))
+        ;; centre-width is inherited from middle child
+        (setf cwidth (undo-tree-node-cwidth (car p)))
+        ;; compute right-width
+        (incf rwidth (undo-tree-node-rwidth (car p)))
+        (setq p (cdr p))
+        (dotimes (i (/ num-children 2))
+          (if (undo-tree-node-lwidth (car p))
+              (incf rwidth (+ (undo-tree-node-lwidth (car p))
+                              (undo-tree-node-cwidth (car p))
+                              (undo-tree-node-rwidth (car p))))
+            (throw 'need-widths (car p)))
+          (setq p (cdr p))))
+
+       ;; even number of children
+       (t
+        (setq p (undo-tree-node-next node))
+        ;; compute left-width
+        (dotimes (i (/ num-children 2))
+          (if (undo-tree-node-lwidth (car p))
+              (incf lwidth (+ (undo-tree-node-lwidth (car p))
+                              (undo-tree-node-cwidth (car p))
+                              (undo-tree-node-rwidth (car p))))
+            (throw 'need-widths (car p)))
+          (setq p (cdr p)))
+        ;; centre-width is 0 when number of children is even
+        (setq cwidth 0)
+        ;; compute right-width
+        (dotimes (i (/ num-children 2))
+          (if (undo-tree-node-lwidth (car p))
+              (incf rwidth (+ (undo-tree-node-lwidth (car p))
+                              (undo-tree-node-cwidth (car p))
+                              (undo-tree-node-rwidth (car p))))
+            (throw 'need-widths (car p)))
+          (setq p (cdr p)))))
+
+      ;; return left-, centre- and right-widths
+      (vector lwidth cwidth rwidth))))
+
+
+(defun undo-tree-clear-visualizer-data (tree)
+  ;; Clear visualizer data below NODE.
+  (undo-tree-mapc
+   (lambda (n) (undo-tree-node-clear-visualizer-data n))
+   (undo-tree-root tree)))
+
+
+(defun undo-tree-node-unmodified-p (node &optional mtime)
+  ;; Return non-nil if NODE corresponds to a buffer state that once upon a
+  ;; time was unmodified. If a file modification time MTIME is specified,
+  ;; return non-nil if the corresponding buffer state really is unmodified.
+  (let (changeset ntime)
+    (setq changeset
+	  (or (undo-tree-node-redo node)
+	      (and (setq changeset (car (undo-tree-node-next node)))
+		   (undo-tree-node-undo changeset)))
+	  ntime
+	  (catch 'found
+	    (dolist (elt changeset)
+	      (when (and (consp elt) (eq (car elt) t) (consp (cdr elt))
+			 (throw 'found (cdr elt)))))))
+    (and ntime
+	 (or (null mtime)
+	     ;; high-precision timestamps
+	     (if (listp (cdr ntime))
+		 (equal ntime mtime)
+	       ;; old-style timestamps
+	       (and (= (car ntime) (car mtime))
+		    (= (cdr ntime) (cadr mtime))))))))
+
+
+
+
+;;; =====================================================================
+;;;                  Undo-in-region utility functions
+
+;; `undo-elt-in-region' uses this as a dynamically-scoped variable
+(defvar undo-adjusted-markers nil)
+
+
+(defun undo-tree-pull-undo-in-region-branch (start end)
+  ;; Pull out entries from undo changesets to create a new undo-in-region
+  ;; branch, which undoes changeset entries lying between START and END first,
+  ;; followed by remaining entries from the changesets, before rejoining the
+  ;; existing undo tree history. Repeated calls will, if appropriate, extend
+  ;; the current undo-in-region branch rather than creating a new one.
+
+  ;; if we're just reverting the last redo-in-region, we don't need to
+  ;; manipulate the undo tree at all
+  (if (undo-tree-reverting-redo-in-region-p start end)
+      t  ; return t to indicate success
+
+    ;; We build the `region-changeset' and `delta-list' lists forwards, using
+    ;; pointers `r' and `d' to the penultimate element of the list. So that we
+    ;; don't have to treat the first element differently, we prepend a dummy
+    ;; leading nil to the lists, and have the pointers point to that
+    ;; initially.
+    ;; Note: using '(nil) instead of (list nil) in the `let*' results in
+    ;;       bizarre errors when the code is byte-compiled, where parts of the
+    ;;       lists appear to survive across different calls to this function.
+    ;;       An obscure byte-compiler bug, perhaps?
+    (let* ((region-changeset (list nil))
+	   (r region-changeset)
+	   (delta-list (list nil))
+	   (d delta-list)
+	   (node (undo-tree-current buffer-undo-tree))
+	   (repeated-undo-in-region
+	    (undo-tree-repeated-undo-in-region-p start end))
+	   undo-adjusted-markers  ; `undo-elt-in-region' expects this
+	   fragment splice original-fragment original-splice original-current
+	   got-visible-elt undo-list elt)
+
+      ;; --- initialisation ---
+      (cond
+       ;; if this is a repeated undo in the same region, start pulling changes
+       ;; from NODE at which undo-in-region branch iss attached, and detatch
+       ;; the branch, using it as initial FRAGMENT of branch being constructed
+       (repeated-undo-in-region
+	(setq original-current node
+	      fragment (car (undo-tree-node-next node))
+	      splice node)
+	;; undo up to node at which undo-in-region branch is attached
+	;; (recognizable as first node with more than one branch)
+	(let ((mark-active nil))
+	  (while (= (length (undo-tree-node-next node)) 1)
+	    (undo-tree-undo-1)
+	    (setq fragment node
+		  node (undo-tree-current buffer-undo-tree))))
+	(when (eq splice node) (setq splice nil))
+	;; detatch undo-in-region branch
+	(setf (undo-tree-node-next node)
+	      (delq fragment (undo-tree-node-next node))
+	      (undo-tree-node-previous fragment) nil
+	      original-fragment fragment
+	      original-splice node))
+
+       ;; if this is a new undo-in-region, initial FRAGMENT is a copy of all
+       ;; nodes below the current one in the active branch
+       ((undo-tree-node-next node)
+	(setq fragment (undo-tree-make-node nil nil)
+	      splice fragment)
+	(while (setq node (nth (undo-tree-node-branch node)
+			       (undo-tree-node-next node)))
+	  (push (undo-tree-make-node
+		 splice
+		 (undo-copy-list (undo-tree-node-undo node))
+		 (undo-copy-list (undo-tree-node-redo node)))
+		(undo-tree-node-next splice))
+	  (setq splice (car (undo-tree-node-next splice))))
+	(setq fragment (car (undo-tree-node-next fragment))
+	      splice nil
+	      node (undo-tree-current buffer-undo-tree))))
+
+
+      ;; --- pull undo-in-region elements into branch ---
+      ;; work backwards up tree, pulling out undo elements within region until
+      ;; we've got one that undoes a visible change (insertion or deletion)
+      (catch 'abort
+	(while (and (not got-visible-elt) node (undo-tree-node-undo node))
+	  ;; we cons a dummy nil element on the front of the changeset so that
+	  ;; we can conveniently remove the first (real) element from the
+	  ;; changeset if we need to; the leading nil is removed once we're
+	  ;; done with this changeset
+	  (setq undo-list (cons nil (undo-copy-list (undo-tree-node-undo node)))
+		elt (cadr undo-list))
+	  (if fragment
+	      (progn
+		(setq fragment (undo-tree-grow-backwards fragment undo-list))
+		(unless splice (setq splice fragment)))
+	    (setq fragment (undo-tree-make-node nil undo-list))
+	    (setq splice fragment))
+
+	  (while elt
+	    (cond
+	     ;; keep elements within region
+	     ((undo-elt-in-region elt start end)
+	      ;; set flag if kept element is visible (insertion or deletion)
+	      (when (and (consp elt)
+			 (or (stringp (car elt)) (integerp (car elt))))
+		(setq got-visible-elt t))
+	      ;; adjust buffer positions in elements previously undone before
+	      ;; kept element, as kept element will now be undone first
+	      (undo-tree-adjust-elements-to-elt splice elt)
+	      ;; move kept element to undo-in-region changeset, adjusting its
+	      ;; buffer position as it will now be undone first
+	      (setcdr r (list (undo-tree-apply-deltas elt (cdr delta-list))))
+	      (setq r (cdr r))
+	      (setcdr undo-list (cddr undo-list)))
+
+	     ;; discard "was unmodified" elements
+	     ;; FIXME: deal properly with these
+	     ((and (consp elt) (eq (car elt) t))
+	      (setcdr undo-list (cddr undo-list)))
+
+	     ;; if element crosses region, we can't pull any more elements
+	     ((undo-elt-crosses-region elt start end)
+	      ;; if we've found a visible element, it must be earlier in
+	      ;; current node's changeset; stop pulling elements (null
+	      ;; `undo-list' and non-nil `got-visible-elt' cause loop to exit)
+	      (if got-visible-elt
+		  (setq undo-list nil)
+		;; if we haven't found a visible element yet, pulling
+		;; undo-in-region branch has failed
+		(setq region-changeset nil)
+		(throw 'abort t)))
+
+	     ;; if rejecting element, add its delta (if any) to the list
+	     (t
+	      (let ((delta (undo-delta elt)))
+		(when (/= 0 (cdr delta))
+		  (setcdr d (list delta))
+		  (setq d (cdr d))))
+	      (setq undo-list (cdr undo-list))))
+
+	    ;; process next element of current changeset
+	    (setq elt (cadr undo-list)))
+
+	  ;; if there are remaining elements in changeset, remove dummy nil
+	  ;; from front
+	  (if (cadr (undo-tree-node-undo fragment))
+	      (pop (undo-tree-node-undo fragment))
+	    ;; otherwise, if we've kept all elements in changeset, discard
+	    ;; empty changeset
+	    (when (eq splice fragment) (setq splice nil))
+	    (setq fragment (car (undo-tree-node-next fragment))))
+	  ;; process changeset from next node up the tree
+	  (setq node (undo-tree-node-previous node))))
+
+      ;; pop dummy nil from front of `region-changeset'
+      (setq region-changeset (cdr region-changeset))
+
+
+      ;; --- integrate branch into tree ---
+      ;; if no undo-in-region elements were found, restore undo tree
+      (if (null region-changeset)
+	  (when original-current
+	    (push original-fragment (undo-tree-node-next original-splice))
+	    (setf (undo-tree-node-branch original-splice) 0
+		  (undo-tree-node-previous original-fragment) original-splice)
+	    (let ((mark-active nil))
+	      (while (not (eq (undo-tree-current buffer-undo-tree)
+			      original-current))
+		(undo-tree-redo-1)))
+	    nil)  ; return nil to indicate failure
+
+	;; otherwise...
+	;; need to undo up to node where new branch will be attached, to
+	;; ensure redo entries are populated, and then redo back to where we
+	;; started
+	(let ((mark-active nil)
+	      (current (undo-tree-current buffer-undo-tree)))
+	  (while (not (eq (undo-tree-current buffer-undo-tree) node))
+	    (undo-tree-undo-1))
+	  (while (not (eq (undo-tree-current buffer-undo-tree) current))
+	    (undo-tree-redo-1)))
+
+	(cond
+	 ;; if there's no remaining fragment, just create undo-in-region node
+	 ;; and attach it to parent of last node from which elements were
+	 ;; pulled
+	 ((null fragment)
+	  (setq fragment (undo-tree-make-node node region-changeset))
+	  (push fragment (undo-tree-node-next node))
+	  (setf (undo-tree-node-branch node) 0)
+	  ;; set current node to undo-in-region node
+	  (setf (undo-tree-current buffer-undo-tree) fragment))
+
+	 ;; if no splice point has been set, add undo-in-region node to top of
+	 ;; fragment and attach it to parent of last node from which elements
+	 ;; were pulled
+	 ((null splice)
+	  (setq fragment (undo-tree-grow-backwards fragment region-changeset))
+	  (push fragment (undo-tree-node-next node))
+	  (setf (undo-tree-node-branch node) 0
+		(undo-tree-node-previous fragment) node)
+	  ;; set current node to undo-in-region node
+	  (setf (undo-tree-current buffer-undo-tree) fragment))
+
+	 ;; if fragment contains nodes, attach fragment to parent of last node
+	 ;; from which elements were pulled, and splice in undo-in-region node
+	 (t
+	  (setf (undo-tree-node-previous fragment) node)
+	  (push fragment (undo-tree-node-next node))
+	  (setf (undo-tree-node-branch node) 0)
+	  ;; if this is a repeated undo-in-region, then we've left the current
+	  ;; node at the original splice-point; we need to set the current
+	  ;; node to the equivalent node on the undo-in-region branch and redo
+	  ;; back to where we started
+	  (when repeated-undo-in-region
+	    (setf (undo-tree-current buffer-undo-tree)
+		  (undo-tree-node-previous original-fragment))
+	    (let ((mark-active nil))
+	      (while (not (eq (undo-tree-current buffer-undo-tree) splice))
+		(undo-tree-redo-1 nil 'preserve-undo))))
+	  ;; splice new undo-in-region node into fragment
+	  (setq node (undo-tree-make-node nil region-changeset))
+	  (undo-tree-splice-node node splice)
+	  ;; set current node to undo-in-region node
+	  (setf (undo-tree-current buffer-undo-tree) node)))
+
+	;; update undo-tree size
+	(setq node (undo-tree-node-previous fragment))
+	(while (progn
+		 (and (setq node (car (undo-tree-node-next node)))
+		      (not (eq node original-fragment))
+		      (incf (undo-tree-count buffer-undo-tree))
+		      (incf (undo-tree-size buffer-undo-tree)
+			    (+ (undo-list-byte-size (undo-tree-node-undo node))
+			       (undo-list-byte-size (undo-tree-node-redo node)))))))
+	t)  ; indicate undo-in-region branch was successfully pulled
+      )))
+
+
+
+(defun undo-tree-pull-redo-in-region-branch (start end)
+  ;; Pull out entries from redo changesets to create a new redo-in-region
+  ;; branch, which redoes changeset entries lying between START and END first,
+  ;; followed by remaining entries from the changesets. Repeated calls will,
+  ;; if appropriate, extend the current redo-in-region branch rather than
+  ;; creating a new one.
+
+  ;; if we're just reverting the last undo-in-region, we don't need to
+  ;; manipulate the undo tree at all
+  (if (undo-tree-reverting-undo-in-region-p start end)
+      t  ; return t to indicate success
+
+    ;; We build the `region-changeset' and `delta-list' lists forwards, using
+    ;; pointers `r' and `d' to the penultimate element of the list. So that we
+    ;; don't have to treat the first element differently, we prepend a dummy
+    ;; leading nil to the lists, and have the pointers point to that
+    ;; initially.
+    ;; Note: using '(nil) instead of (list nil) in the `let*' causes bizarre
+    ;;       errors when the code is byte-compiled, where parts of the lists
+    ;;       appear to survive across different calls to this function.  An
+    ;;       obscure byte-compiler bug, perhaps?
+    (let* ((region-changeset (list nil))
+	   (r region-changeset)
+	   (delta-list (list nil))
+	   (d delta-list)
+	   (node (undo-tree-current buffer-undo-tree))
+	   (repeated-redo-in-region
+	    (undo-tree-repeated-redo-in-region-p start end))
+	   undo-adjusted-markers  ; `undo-elt-in-region' expects this
+	   fragment splice got-visible-elt redo-list elt)
+
+      ;; --- inisitalisation ---
+      (cond
+       ;; if this is a repeated redo-in-region, detach fragment below current
+       ;; node
+       (repeated-redo-in-region
+	(when (setq fragment (car (undo-tree-node-next node)))
+	  (setf (undo-tree-node-previous fragment) nil
+		(undo-tree-node-next node)
+		(delq fragment (undo-tree-node-next node)))))
+       ;; if this is a new redo-in-region, initial fragment is a copy of all
+       ;; nodes below the current one in the active branch
+       ((undo-tree-node-next node)
+	(setq fragment (undo-tree-make-node nil nil)
+	      splice fragment)
+	(while (setq node (nth (undo-tree-node-branch node)
+			       (undo-tree-node-next node)))
+	  (push (undo-tree-make-node
+		 splice nil
+		 (undo-copy-list (undo-tree-node-redo node)))
+		(undo-tree-node-next splice))
+	  (setq splice (car (undo-tree-node-next splice))))
+	(setq fragment (car (undo-tree-node-next fragment)))))
+
+
+      ;; --- pull redo-in-region elements into branch ---
+      ;; work down fragment, pulling out redo elements within region until
+      ;; we've got one that redoes a visible change (insertion or deletion)
+      (setq node fragment)
+      (catch 'abort
+	(while (and (not got-visible-elt) node (undo-tree-node-redo node))
+	  ;; we cons a dummy nil element on the front of the changeset so that
+	  ;; we can conveniently remove the first (real) element from the
+	  ;; changeset if we need to; the leading nil is removed once we're
+	  ;; done with this changeset
+	  (setq redo-list (push nil (undo-tree-node-redo node))
+		elt (cadr redo-list))
+	  (while elt
+	    (cond
+	     ;; keep elements within region
+	     ((undo-elt-in-region elt start end)
+	      ;; set flag if kept element is visible (insertion or deletion)
+	      (when (and (consp elt)
+			 (or (stringp (car elt)) (integerp (car elt))))
+		(setq got-visible-elt t))
+	      ;; adjust buffer positions in elements previously redone before
+	      ;; kept element, as kept element will now be redone first
+	      (undo-tree-adjust-elements-to-elt fragment elt t)
+	      ;; move kept element to redo-in-region changeset, adjusting its
+	      ;; buffer position as it will now be redone first
+	      (setcdr r (list (undo-tree-apply-deltas elt (cdr delta-list) -1)))
+	      (setq r (cdr r))
+	      (setcdr redo-list (cddr redo-list)))
+
+	     ;; discard "was unmodified" elements
+	     ;; FIXME: deal properly with these
+	     ((and (consp elt) (eq (car elt) t))
+	      (setcdr redo-list (cddr redo-list)))
+
+	     ;; if element crosses region, we can't pull any more elements
+	     ((undo-elt-crosses-region elt start end)
+	      ;; if we've found a visible element, it must be earlier in
+	      ;; current node's changeset; stop pulling elements (null
+	      ;; `redo-list' and non-nil `got-visible-elt' cause loop to exit)
+	      (if got-visible-elt
+		  (setq redo-list nil)
+		;; if we haven't found a visible element yet, pulling
+		;; redo-in-region branch has failed
+		(setq region-changeset nil)
+		(throw 'abort t)))
+
+	     ;; if rejecting element, add its delta (if any) to the list
+	     (t
+	      (let ((delta (undo-delta elt)))
+		(when (/= 0 (cdr delta))
+		  (setcdr d (list delta))
+		  (setq d (cdr d))))
+	      (setq redo-list (cdr redo-list))))
+
+	    ;; process next element of current changeset
+	    (setq elt (cadr redo-list)))
+
+	  ;; if there are remaining elements in changeset, remove dummy nil
+	  ;; from front
+	  (if (cadr (undo-tree-node-redo node))
+	      (pop (undo-tree-node-undo node))
+	    ;; otherwise, if we've kept all elements in changeset, discard
+	    ;; empty changeset
+	    (if (eq fragment node)
+		(setq fragment (car (undo-tree-node-next fragment)))
+	      (undo-tree-snip-node node)))
+	  ;; process changeset from next node in fragment
+	  (setq node (car (undo-tree-node-next node)))))
+
+      ;; pop dummy nil from front of `region-changeset'
+      (setq region-changeset (cdr region-changeset))
+
+
+      ;; --- integrate branch into tree ---
+      (setq node (undo-tree-current buffer-undo-tree))
+      ;; if no redo-in-region elements were found, restore undo tree
+      (if (null (car region-changeset))
+	  (when (and repeated-redo-in-region fragment)
+	    (push fragment (undo-tree-node-next node))
+	    (setf (undo-tree-node-branch node) 0
+		  (undo-tree-node-previous fragment) node)
+	    nil)  ; return nil to indicate failure
+
+	;; otherwise, add redo-in-region node to top of fragment, and attach
+	;; it below current node
+	(setq fragment
+	      (if fragment
+		  (undo-tree-grow-backwards fragment nil region-changeset)
+		(undo-tree-make-node nil nil region-changeset)))
+	(push fragment (undo-tree-node-next node))
+	(setf (undo-tree-node-branch node) 0
+	      (undo-tree-node-previous fragment) node)
+	;; update undo-tree size
+	(unless repeated-redo-in-region
+	  (setq node fragment)
+	  (while (and (setq node (car (undo-tree-node-next node)))
+		      (incf (undo-tree-count buffer-undo-tree))
+		      (incf (undo-tree-size buffer-undo-tree)
+			    (undo-list-byte-size
+			     (undo-tree-node-redo node))))))
+	(incf (undo-tree-size buffer-undo-tree)
+	      (undo-list-byte-size (undo-tree-node-redo fragment)))
+	t)  ; indicate redo-in-region branch was successfully pulled
+      )))
+
+
+
+(defun undo-tree-adjust-elements-to-elt (node undo-elt &optional below)
+  "Adjust buffer positions of undo elements, starting at NODE's
+and going up the tree (or down the active branch if BELOW is
+non-nil) and through the nodes' undo elements until we reach
+UNDO-ELT.  UNDO-ELT must appear somewhere in the undo changeset
+of either NODE itself or some node above it in the tree."
+  (let ((delta (list (undo-delta undo-elt)))
+	(undo-list (undo-tree-node-undo node)))
+    ;; adjust elements until we reach UNDO-ELT
+    (while (and (car undo-list)
+		(not (eq (car undo-list) undo-elt)))
+      (setcar undo-list
+	      (undo-tree-apply-deltas (car undo-list) delta -1))
+      ;; move to next undo element in list, or to next node if we've run out
+      ;; of elements
+      (unless (car (setq undo-list (cdr undo-list)))
+	(if below
+	    (setq node (nth (undo-tree-node-branch node)
+			    (undo-tree-node-next node)))
+	  (setq node (undo-tree-node-previous node)))
+	(setq undo-list (undo-tree-node-undo node))))))
+
+
+
+(defun undo-tree-apply-deltas (undo-elt deltas &optional sgn)
+  ;; Apply DELTAS in order to UNDO-ELT, multiplying deltas by SGN
+  ;; (only useful value for SGN is -1).
+  (let (position offset)
+    (dolist (delta deltas)
+      (setq position (car delta)
+	    offset (* (cdr delta) (or sgn 1)))
+      (cond
+       ;; POSITION
+       ((integerp undo-elt)
+	(when (>= undo-elt position)
+	  (setq undo-elt (- undo-elt offset))))
+       ;; nil (or any other atom)
+       ((atom undo-elt))
+       ;; (TEXT . POSITION)
+       ((stringp (car undo-elt))
+	(let ((text-pos (abs (cdr undo-elt)))
+	      (point-at-end (< (cdr undo-elt) 0)))
+	  (if (>= text-pos position)
+	      (setcdr undo-elt (* (if point-at-end -1 1)
+				  (- text-pos offset))))))
+       ;; (BEGIN . END)
+       ((integerp (car undo-elt))
+	(when (>= (car undo-elt) position)
+	  (setcar undo-elt (- (car undo-elt) offset))
+	  (setcdr undo-elt (- (cdr undo-elt) offset))))
+       ;; (nil PROPERTY VALUE BEG . END)
+       ((null (car undo-elt))
+	(let ((tail (nthcdr 3 undo-elt)))
+	  (when (>= (car tail) position)
+	    (setcar tail (- (car tail) offset))
+	    (setcdr tail (- (cdr tail) offset)))))
+       ))
+    undo-elt))
+
+
+
+(defun undo-tree-repeated-undo-in-region-p (start end)
+  ;; Return non-nil if undo-in-region between START and END is a repeated
+  ;; undo-in-region
+  (let ((node (undo-tree-current buffer-undo-tree)))
+    (and (setq node
+	       (nth (undo-tree-node-branch node) (undo-tree-node-next node)))
+	 (eq (undo-tree-node-undo-beginning node) start)
+	 (eq (undo-tree-node-undo-end node) end))))
+
+
+(defun undo-tree-repeated-redo-in-region-p (start end)
+  ;; Return non-nil if undo-in-region between START and END is a repeated
+  ;; undo-in-region
+  (let ((node (undo-tree-current buffer-undo-tree)))
+    (and (eq (undo-tree-node-redo-beginning node) start)
+	 (eq (undo-tree-node-redo-end node) end))))
+
+
+;; Return non-nil if undo-in-region between START and END is simply
+;; reverting the last redo-in-region
+(defalias 'undo-tree-reverting-undo-in-region-p
+  'undo-tree-repeated-undo-in-region-p)
+
+
+;; Return non-nil if redo-in-region between START and END is simply
+;; reverting the last undo-in-region
+(defalias 'undo-tree-reverting-redo-in-region-p
+  'undo-tree-repeated-redo-in-region-p)
+
+
+
+
+;;; =====================================================================
+;;;                        Undo-tree commands
+
+;;;###autoload
+(define-minor-mode undo-tree-mode
+  "Toggle undo-tree mode.
+With no argument, this command toggles the mode.
+A positive prefix argument turns the mode on.
+A negative prefix argument turns it off.
+
+Undo-tree-mode replaces Emacs' standard undo feature with a more
+powerful yet easier to use version, that treats the undo history
+as what it is: a tree.
+
+The following keys are available in `undo-tree-mode':
+
+  \\{undo-tree-map}
+
+Within the undo-tree visualizer, the following keys are available:
+
+  \\{undo-tree-visualizer-mode-map}"
+
+  nil                       ; init value
+  undo-tree-mode-lighter    ; lighter
+  undo-tree-map             ; keymap
+
+  ;; if disabling `undo-tree-mode', rebuild `buffer-undo-list' from tree so
+  ;; Emacs undo can work
+  (when (not undo-tree-mode)
+    (undo-list-rebuild-from-tree)
+    (setq buffer-undo-tree nil)))
+
+
+(defun turn-on-undo-tree-mode (&optional print-message)
+  "Enable `undo-tree-mode' in the current buffer, when appropriate.
+Some major modes implement their own undo system, which should
+not normally be overridden by `undo-tree-mode'. This command does
+not enable `undo-tree-mode' in such buffers. If you want to force
+`undo-tree-mode' to be enabled regardless, use (undo-tree-mode 1)
+instead.
+
+The heuristic used to detect major modes in which
+`undo-tree-mode' should not be used is to check whether either
+the `undo' command has been remapped, or the default undo
+keybindings (C-/ and C-_) have been overridden somewhere other
+than in the global map. In addition, `undo-tree-mode' will not be
+enabled if the buffer's `major-mode' appears in
+`undo-tree-incompatible-major-modes'."
+  (interactive "p")
+  (if (or (key-binding [remap undo])
+	  (undo-tree-overridden-undo-bindings-p)
+	  (memq major-mode undo-tree-incompatible-major-modes))
+      (when print-message
+	(message "Buffer does not support undo-tree-mode;\
+ undo-tree-mode NOT enabled"))
+    (undo-tree-mode 1)))
+
+
+(defun undo-tree-overridden-undo-bindings-p ()
+  "Returns t if default undo bindings are overridden, nil otherwise.
+Checks if either of the default undo key bindings (\"C-/\" or
+\"C-_\") are overridden in the current buffer by any keymap other
+than the global one. (So global redefinitions of the default undo
+key bindings do not count.)"
+  (let ((binding1 (lookup-key (current-global-map) [?\C-/]))
+	(binding2 (lookup-key (current-global-map) [?\C-_])))
+    (global-set-key [?\C-/] 'undo)
+    (global-set-key [?\C-_] 'undo)
+    (unwind-protect
+	(or (and (key-binding [?\C-/])
+		 (not (eq (key-binding [?\C-/]) 'undo)))
+	    (and (key-binding [?\C-_])
+		 (not (eq (key-binding [?\C-_]) 'undo))))
+      (global-set-key [?\C-/] binding1)
+      (global-set-key [?\C-_] binding2))))
+
+
+;;;###autoload
+(define-globalized-minor-mode global-undo-tree-mode
+  undo-tree-mode turn-on-undo-tree-mode)
+
+
+
+(defun undo-tree-undo (&optional arg)
+  "Undo changes.
+Repeat this command to undo more changes.
+A numeric ARG serves as a repeat count.
+
+In Transient Mark mode when the mark is active, only undo changes
+within the current region. Similarly, when not in Transient Mark
+mode, just \\[universal-argument] as an argument limits undo to
+changes within the current region."
+  (interactive "*P")
+  ;; throw error if undo is disabled in buffer
+  (when (eq buffer-undo-list t)
+    (user-error "No undo information in this buffer"))
+  (undo-tree-undo-1 arg)
+  ;; inform user if at branch point
+  (when (> (undo-tree-num-branches) 1) (message "Undo branch point!")))
+
+
+(defun undo-tree-undo-1 (&optional arg preserve-redo preserve-timestamps)
+  ;; Internal undo function. An active mark in `transient-mark-mode', or
+  ;; non-nil ARG otherwise, enables undo-in-region. Non-nil PRESERVE-REDO
+  ;; causes the existing redo record to be preserved, rather than replacing it
+  ;; with the new one generated by undoing. Non-nil PRESERVE-TIMESTAMPS
+  ;; disables updating of timestamps in visited undo-tree nodes. (This latter
+  ;; should *only* be used when temporarily visiting another undo state and
+  ;; immediately returning to the original state afterwards. Otherwise, it
+  ;; could cause history-discarding errors.)
+  (let ((undo-in-progress t)
+	(undo-in-region (and undo-tree-enable-undo-in-region
+			     (or (region-active-p)
+				 (and arg (not (numberp arg))))))
+	pos current)
+    ;; transfer entries accumulated in `buffer-undo-list' to
+    ;; `buffer-undo-tree'
+    (undo-list-transfer-to-tree)
+
+    (dotimes (i (or (and (numberp arg) (prefix-numeric-value arg)) 1))
+      ;; check if at top of undo tree
+      (unless (undo-tree-node-previous (undo-tree-current buffer-undo-tree))
+	(user-error "No further undo information"))
+
+      ;; if region is active, or a non-numeric prefix argument was supplied,
+      ;; try to pull out a new branch of changes affecting the region
+      (when (and undo-in-region
+		 (not (undo-tree-pull-undo-in-region-branch
+		       (region-beginning) (region-end))))
+	(user-error "No further undo information for region"))
+
+      ;; remove any GC'd elements from node's undo list
+      (setq current (undo-tree-current buffer-undo-tree))
+      (decf (undo-tree-size buffer-undo-tree)
+	    (undo-list-byte-size (undo-tree-node-undo current)))
+      (setf (undo-tree-node-undo current)
+	    (undo-list-clean-GCd-elts (undo-tree-node-undo current)))
+      (incf (undo-tree-size buffer-undo-tree)
+	    (undo-list-byte-size (undo-tree-node-undo current)))
+      ;; undo one record from undo tree
+      (when undo-in-region
+	(setq pos (set-marker (make-marker) (point)))
+	(set-marker-insertion-type pos t))
+      (primitive-undo 1 (undo-tree-copy-list (undo-tree-node-undo current)))
+      (undo-boundary)
+
+      ;; if preserving old redo record, discard new redo entries that
+      ;; `primitive-undo' has added to `buffer-undo-list', and remove any GC'd
+      ;; elements from node's redo list
+      (if preserve-redo
+	  (progn
+	    (undo-list-pop-changeset)
+	    (decf (undo-tree-size buffer-undo-tree)
+		  (undo-list-byte-size (undo-tree-node-redo current)))
+	    (setf (undo-tree-node-redo current)
+		  (undo-list-clean-GCd-elts (undo-tree-node-redo current)))
+	    (incf (undo-tree-size buffer-undo-tree)
+		  (undo-list-byte-size (undo-tree-node-redo current))))
+	;; otherwise, record redo entries that `primitive-undo' has added to
+	;; `buffer-undo-list' in current node's redo record, replacing
+	;; existing entry if one already exists
+	(decf (undo-tree-size buffer-undo-tree)
+	      (undo-list-byte-size (undo-tree-node-redo current)))
+	(setf (undo-tree-node-redo current)
+	      (undo-list-pop-changeset 'discard-pos))
+	(incf (undo-tree-size buffer-undo-tree)
+	      (undo-list-byte-size (undo-tree-node-redo current))))
+
+      ;; rewind current node and update timestamp
+      (setf (undo-tree-current buffer-undo-tree)
+	    (undo-tree-node-previous (undo-tree-current buffer-undo-tree)))
+      (unless preserve-timestamps
+	(setf (undo-tree-node-timestamp (undo-tree-current buffer-undo-tree))
+	      (current-time)))
+
+      ;; if undoing-in-region, record current node, region and direction so we
+      ;; can tell if undo-in-region is repeated, and re-activate mark if in
+      ;; `transient-mark-mode'; if not, erase any leftover data
+      (if (not undo-in-region)
+	  (undo-tree-node-clear-region-data current)
+	(goto-char pos)
+	;; note: we deliberately want to store the region information in the
+	;; node *below* the now current one
+	(setf (undo-tree-node-undo-beginning current) (region-beginning)
+	      (undo-tree-node-undo-end current) (region-end))
+	(set-marker pos nil)))
+
+    ;; undo deactivates mark unless undoing-in-region
+    (setq deactivate-mark (not undo-in-region))))
+
+
+
+(defun undo-tree-redo (&optional arg)
+  "Redo changes. A numeric ARG serves as a repeat count.
+
+In Transient Mark mode when the mark is active, only redo changes
+within the current region. Similarly, when not in Transient Mark
+mode, just \\[universal-argument] as an argument limits redo to
+changes within the current region."
+  (interactive "*P")
+  ;; throw error if undo is disabled in buffer
+  (when (eq buffer-undo-list t)
+    (user-error "No undo information in this buffer"))
+  (undo-tree-redo-1 arg)
+  ;; inform user if at branch point
+  (when (> (undo-tree-num-branches) 1) (message "Undo branch point!")))
+
+
+(defun undo-tree-redo-1 (&optional arg preserve-undo preserve-timestamps)
+  ;; Internal redo function. An active mark in `transient-mark-mode', or
+  ;; non-nil ARG otherwise, enables undo-in-region. Non-nil PRESERVE-UNDO
+  ;; causes the existing redo record to be preserved, rather than replacing it
+  ;; with the new one generated by undoing. Non-nil PRESERVE-TIMESTAMPS
+  ;; disables updating of timestamps in visited undo-tree nodes. (This latter
+  ;; should *only* be used when temporarily visiting another undo state and
+  ;; immediately returning to the original state afterwards. Otherwise, it
+  ;; could cause history-discarding errors.)
+  (let ((undo-in-progress t)
+	(redo-in-region (and undo-tree-enable-undo-in-region
+			     (or (region-active-p)
+				 (and arg (not (numberp arg))))))
+	pos current)
+    ;; transfer entries accumulated in `buffer-undo-list' to
+    ;; `buffer-undo-tree'
+    (undo-list-transfer-to-tree)
+
+    (dotimes (i (or (and (numberp arg) (prefix-numeric-value arg)) 1))
+      ;; check if at bottom of undo tree
+      (when (null (undo-tree-node-next (undo-tree-current buffer-undo-tree)))
+	(user-error "No further redo information"))
+
+      ;; if region is active, or a non-numeric prefix argument was supplied,
+      ;; try to pull out a new branch of changes affecting the region
+      (when (and redo-in-region
+		 (not (undo-tree-pull-redo-in-region-branch
+		       (region-beginning) (region-end))))
+	(user-error "No further redo information for region"))
+
+      ;; get next node (but DON'T advance current node in tree yet, in case
+      ;; redoing fails)
+      (setq current (undo-tree-current buffer-undo-tree)
+	    current (nth (undo-tree-node-branch current)
+			 (undo-tree-node-next current)))
+      ;; remove any GC'd elements from node's redo list
+      (decf (undo-tree-size buffer-undo-tree)
+	    (undo-list-byte-size (undo-tree-node-redo current)))
+      (setf (undo-tree-node-redo current)
+	    (undo-list-clean-GCd-elts (undo-tree-node-redo current)))
+      (incf (undo-tree-size buffer-undo-tree)
+	    (undo-list-byte-size (undo-tree-node-redo current)))
+      ;; redo one record from undo tree
+      (when redo-in-region
+	(setq pos (set-marker (make-marker) (point)))
+	(set-marker-insertion-type pos t))
+      (primitive-undo 1 (undo-tree-copy-list (undo-tree-node-redo current)))
+      (undo-boundary)
+      ;; advance current node in tree
+      (setf (undo-tree-current buffer-undo-tree) current)
+
+      ;; if preserving old undo record, discard new undo entries that
+      ;; `primitive-undo' has added to `buffer-undo-list', and remove any GC'd
+      ;; elements from node's redo list
+      (if preserve-undo
+	  (progn
+	    (undo-list-pop-changeset)
+	    (decf (undo-tree-size buffer-undo-tree)
+		  (undo-list-byte-size (undo-tree-node-undo current)))
+	    (setf (undo-tree-node-undo current)
+		  (undo-list-clean-GCd-elts (undo-tree-node-undo current)))
+	    (incf (undo-tree-size buffer-undo-tree)
+		  (undo-list-byte-size (undo-tree-node-undo current))))
+	;; otherwise, record undo entries that `primitive-undo' has added to
+	;; `buffer-undo-list' in current node's undo record, replacing
+	;; existing entry if one already exists
+	(decf (undo-tree-size buffer-undo-tree)
+	      (undo-list-byte-size (undo-tree-node-undo current)))
+	(setf (undo-tree-node-undo current)
+	      (undo-list-pop-changeset 'discard-pos))
+	(incf (undo-tree-size buffer-undo-tree)
+	      (undo-list-byte-size (undo-tree-node-undo current))))
+
+      ;; update timestamp
+      (unless preserve-timestamps
+	(setf (undo-tree-node-timestamp current) (current-time)))
+
+      ;; if redoing-in-region, record current node, region and direction so we
+      ;; can tell if redo-in-region is repeated, and re-activate mark if in
+      ;; `transient-mark-mode'
+      (if (not redo-in-region)
+	  (undo-tree-node-clear-region-data current)
+	(goto-char pos)
+	(setf (undo-tree-node-redo-beginning current) (region-beginning)
+	      (undo-tree-node-redo-end current) (region-end))
+	(set-marker pos nil)))
+
+    ;; redo deactivates the mark unless redoing-in-region
+    (setq deactivate-mark (not redo-in-region))))
+
+
+
+(defun undo-tree-switch-branch (branch)
+  "Switch to a different BRANCH of the undo tree.
+This will affect which branch to descend when *redoing* changes
+using `undo-tree-redo'."
+  (interactive (list (or (and prefix-arg (prefix-numeric-value prefix-arg))
+                         (and (not (eq buffer-undo-list t))
+			      (or (undo-list-transfer-to-tree) t)
+			      (let ((b (undo-tree-node-branch
+					(undo-tree-current
+					 buffer-undo-tree))))
+				(cond
+				 ;; switch to other branch if only 2
+				 ((= (undo-tree-num-branches) 2) (- 1 b))
+				 ;; prompt if more than 2
+				 ((> (undo-tree-num-branches) 2)
+				  (read-number
+				   (format "Branch (0-%d, on %d): "
+					   (1- (undo-tree-num-branches)) b)))
+				 ))))))
+  ;; throw error if undo is disabled in buffer
+  (when (eq buffer-undo-list t)
+    (user-error "No undo information in this buffer"))
+  ;; sanity check branch number
+  (when (<= (undo-tree-num-branches) 1)
+    (user-error "Not at undo branch point"))
+  (when (or (< branch 0) (> branch (1- (undo-tree-num-branches))))
+    (user-error "Invalid branch number"))
+  ;; transfer entries accumulated in `buffer-undo-list' to `buffer-undo-tree'
+  (undo-list-transfer-to-tree)
+  ;; switch branch
+  (setf (undo-tree-node-branch (undo-tree-current buffer-undo-tree))
+	branch)
+  (message "Switched to branch %d" branch))
+
+
+(defun undo-tree-set (node &optional preserve-timestamps)
+  ;; Set buffer to state corresponding to NODE. Returns intersection point
+  ;; between path back from current node and path back from selected NODE.
+  ;; Non-nil PRESERVE-TIMESTAMPS disables updating of timestamps in visited
+  ;; undo-tree nodes. (This should *only* be used when temporarily visiting
+  ;; another undo state and immediately returning to the original state
+  ;; afterwards. Otherwise, it could cause history-discarding errors.)
+  (let ((path (make-hash-table :test 'eq))
+        (n node))
+    (puthash (undo-tree-root buffer-undo-tree) t path)
+    ;; build list of nodes leading back from selected node to root, updating
+    ;; branches as we go to point down to selected node
+    (while (progn
+             (puthash n t path)
+             (when (undo-tree-node-previous n)
+               (setf (undo-tree-node-branch (undo-tree-node-previous n))
+                     (undo-tree-position
+                      n (undo-tree-node-next (undo-tree-node-previous n))))
+               (setq n (undo-tree-node-previous n)))))
+    ;; work backwards from current node until we intersect path back from
+    ;; selected node
+    (setq n (undo-tree-current buffer-undo-tree))
+    (while (not (gethash n path))
+      (setq n (undo-tree-node-previous n)))
+    ;; ascend tree until intersection node
+    (while (not (eq (undo-tree-current buffer-undo-tree) n))
+      (undo-tree-undo-1 nil nil preserve-timestamps))
+    ;; descend tree until selected node
+    (while (not (eq (undo-tree-current buffer-undo-tree) node))
+      (undo-tree-redo-1 nil nil preserve-timestamps))
+    n))  ; return intersection node
+
+
+
+(defun undo-tree-save-state-to-register (register)
+  "Store current undo-tree state to REGISTER.
+The saved state can be restored using
+`undo-tree-restore-state-from-register'.
+Argument is a character, naming the register."
+  (interactive "cUndo-tree state to register: ")
+  ;; throw error if undo is disabled in buffer
+  (when (eq buffer-undo-list t)
+    (user-error "No undo information in this buffer"))
+  ;; transfer entries accumulated in `buffer-undo-list' to `buffer-undo-tree'
+  (undo-list-transfer-to-tree)
+  ;; save current node to REGISTER
+  (set-register
+   register (registerv-make
+	     (undo-tree-make-register-data
+	      (current-buffer) (undo-tree-current buffer-undo-tree))
+	     :print-func 'undo-tree-register-data-print-func))
+  ;; record REGISTER in current node, for visualizer
+  (setf (undo-tree-node-register (undo-tree-current buffer-undo-tree))
+	register))
+
+
+
+(defun undo-tree-restore-state-from-register (register)
+  "Restore undo-tree state from REGISTER.
+The state must be saved using `undo-tree-save-state-to-register'.
+Argument is a character, naming the register."
+  (interactive "*cRestore undo-tree state from register: ")
+  ;; throw error if undo is disabled in buffer, or if register doesn't contain
+  ;; an undo-tree node
+  (let ((data (registerv-data (get-register register))))
+    (cond
+     ((eq buffer-undo-list t)
+      (user-error "No undo information in this buffer"))
+     ((not (undo-tree-register-data-p data))
+      (user-error "Register doesn't contain undo-tree state"))
+     ((not (eq (current-buffer) (undo-tree-register-data-buffer data)))
+      (user-error "Register contains undo-tree state for a different buffer")))
+    ;; transfer entries accumulated in `buffer-undo-list' to `buffer-undo-tree'
+    (undo-list-transfer-to-tree)
+    ;; restore buffer state corresponding to saved node
+    (undo-tree-set (undo-tree-register-data-node data))))
+
+
+
+
+;;; =====================================================================
+;;;                    Persistent storage commands
+
+(defun undo-tree-make-history-save-file-name (file)
+  "Create the undo history file name for FILE.
+Normally this is the file's name with \".\" prepended and
+\".~undo-tree~\" appended.
+
+A match for FILE is sought in `undo-tree-history-directory-alist'
+\(see the documentation of that variable for details\). If the
+directory for the backup doesn't exist, it is created."
+  (let* ((backup-directory-alist undo-tree-history-directory-alist)
+	 (name (make-backup-file-name-1 file)))
+    (concat (file-name-directory name) "." (file-name-nondirectory name)
+	    ".~undo-tree~")))
+
+
+(defun undo-tree-save-history (&optional filename overwrite)
+  "Store undo-tree history to file.
+
+If optional argument FILENAME is omitted, default save file is
+\".<buffer-file-name>.~undo-tree\" if buffer is visiting a file.
+Otherwise, prompt for one.
+
+If OVERWRITE is non-nil, any existing file will be overwritten
+without asking for confirmation."
+  (interactive)
+  (when (eq buffer-undo-list t)
+    (user-error "No undo information in this buffer"))
+  (undo-list-transfer-to-tree)
+  (when (and buffer-undo-tree (not (eq buffer-undo-tree t)))
+    (condition-case nil
+	(undo-tree-kill-visualizer)
+      (error (undo-tree-clear-visualizer-data buffer-undo-tree)))
+    (let ((buff (current-buffer))
+	  tree)
+      ;; get filename
+      (unless filename
+	(setq filename
+	      (if buffer-file-name
+		  (undo-tree-make-history-save-file-name buffer-file-name)
+		(expand-file-name (read-file-name "File to save in: ") nil))))
+      (when (or (not (file-exists-p filename))
+		overwrite
+		(yes-or-no-p (format "Overwrite \"%s\"? " filename)))
+	(unwind-protect
+	    (progn
+	      ;; transform undo-tree into non-circular structure, and make
+	      ;; temporary copy
+	      (undo-tree-decircle buffer-undo-tree)
+	      (setq tree (copy-undo-tree buffer-undo-tree))
+	      ;; discard undo-tree object pool before saving
+	      (setf (undo-tree-object-pool tree) nil)
+	      ;; print undo-tree to file
+	      ;; NOTE: We use `with-temp-buffer' instead of `with-temp-file'
+	      ;;       to allow `auto-compression-mode' to take effect, in
+	      ;;       case user has overridden or advised the default
+	      ;;       `undo-tree-make-history-save-file-name' to add a
+	      ;;       compressed file extension.
+	      (with-auto-compression-mode
+		(with-temp-buffer
+		  (prin1 (sha1 buff) (current-buffer))
+		  (terpri (current-buffer))
+		  (let ((print-circle t)) (prin1 tree (current-buffer)))
+		  (write-region nil nil filename))))
+	  ;; restore circular undo-tree data structure
+	  (undo-tree-recircle buffer-undo-tree))
+	))))
+
+
+
+(defun undo-tree-load-history (&optional filename noerror)
+  "Load undo-tree history from file.
+
+If optional argument FILENAME is null, default load file is
+\".<buffer-file-name>.~undo-tree\" if buffer is visiting a file.
+Otherwise, prompt for one.
+
+If optional argument NOERROR is non-nil, return nil instead of
+signaling an error if file is not found."
+  (interactive)
+  ;; get filename
+  (unless filename
+    (setq filename
+	  (if buffer-file-name
+	      (undo-tree-make-history-save-file-name buffer-file-name)
+	    (expand-file-name (read-file-name "File to load from: ") nil))))
+
+  ;; attempt to read undo-tree from FILENAME
+  (catch 'load-error
+    (unless (file-exists-p filename)
+      (if noerror
+	  (throw 'load-error nil)
+	(error "File \"%s\" does not exist; could not load undo-tree history"
+	       filename)))
+    (let (buff hash tree)
+      (setq buff (current-buffer))
+      (with-auto-compression-mode
+	(with-temp-buffer
+	  (insert-file-contents filename)
+	  (goto-char (point-min))
+	  (condition-case nil
+	      (setq hash (read (current-buffer)))
+	    (error
+	     (kill-buffer nil)
+	     (funcall (if noerror 'message 'user-error)
+		      "Error reading undo-tree history from \"%s\"" filename)
+	     (throw 'load-error nil)))
+	  (unless (string= (sha1 buff) hash)
+	    (kill-buffer nil)
+	    (funcall (if noerror 'message 'user-error)
+		     "Buffer has been modified; could not load undo-tree history")
+	    (throw 'load-error nil))
+	  (condition-case nil
+	      (setq tree (read (current-buffer)))
+	    (error
+	     (kill-buffer nil)
+	     (funcall (if noerror 'message 'error)
+		      "Error reading undo-tree history from \"%s\"" filename)
+	     (throw 'load-error nil)))
+	  (kill-buffer nil)))
+      ;; initialise empty undo-tree object pool
+      (setf (undo-tree-object-pool tree)
+	    (make-hash-table :test 'eq :weakness 'value))
+      ;; restore circular undo-tree data structure
+      (undo-tree-recircle tree)
+      (setq buffer-undo-tree tree))))
+
+
+
+;; Versions of save/load functions for use in hooks
+(defun undo-tree-save-history-hook ()
+  (when (and undo-tree-mode undo-tree-auto-save-history
+	     (not (eq buffer-undo-list t)))
+    (undo-tree-save-history nil t) nil))
+
+(defun undo-tree-load-history-hook ()
+  (when (and undo-tree-mode undo-tree-auto-save-history
+	     (not (eq buffer-undo-list t))
+	     (not revert-buffer-in-progress-p))
+    (undo-tree-load-history nil t)))
+
+
+
+
+;;; =====================================================================
+;;;                    Visualizer drawing functions
+
+(defun undo-tree-visualize ()
+  "Visualize the current buffer's undo tree."
+  (interactive "*")
+  (deactivate-mark)
+  ;; throw error if undo is disabled in buffer
+  (when (eq buffer-undo-list t)
+    (user-error "No undo information in this buffer"))
+  ;; transfer entries accumulated in `buffer-undo-list' to `buffer-undo-tree'
+  (undo-list-transfer-to-tree)
+  ;; add hook to kill visualizer buffer if original buffer is changed
+  (add-hook 'before-change-functions 'undo-tree-kill-visualizer nil t)
+  ;; prepare *undo-tree* buffer, then draw tree in it
+  (let ((undo-tree buffer-undo-tree)
+        (buff (current-buffer))
+	(display-buffer-mark-dedicated 'soft))
+    (switch-to-buffer-other-window
+     (get-buffer-create undo-tree-visualizer-buffer-name))
+    (setq undo-tree-visualizer-parent-buffer buff)
+    (setq undo-tree-visualizer-parent-mtime
+	  (and (buffer-file-name buff)
+	       (nth 5 (file-attributes (buffer-file-name buff)))))
+    (setq undo-tree-visualizer-initial-node (undo-tree-current undo-tree))
+    (setq undo-tree-visualizer-spacing
+	  (undo-tree-visualizer-calculate-spacing))
+    (make-local-variable 'undo-tree-visualizer-timestamps)
+    (make-local-variable 'undo-tree-visualizer-diff)
+    (setq buffer-undo-tree undo-tree)
+    (undo-tree-visualizer-mode)
+    ;; FIXME; don't know why `undo-tree-visualizer-mode' clears this
+    (setq buffer-undo-tree undo-tree)
+    (set (make-local-variable 'undo-tree-visualizer-lazy-drawing)
+	 (or (eq undo-tree-visualizer-lazy-drawing t)
+	     (and (numberp undo-tree-visualizer-lazy-drawing)
+		  (>= (undo-tree-count undo-tree)
+		      undo-tree-visualizer-lazy-drawing))))
+    (when undo-tree-visualizer-diff (undo-tree-visualizer-show-diff))
+    (let ((inhibit-read-only t)) (undo-tree-draw-tree undo-tree))))
+
+
+(defun undo-tree-kill-visualizer (&rest _dummy)
+  ;; Kill visualizer. Added to `before-change-functions' hook of original
+  ;; buffer when visualizer is invoked.
+  (unless (or undo-tree-inhibit-kill-visualizer
+	      (null (get-buffer undo-tree-visualizer-buffer-name)))
+    (with-current-buffer undo-tree-visualizer-buffer-name
+      (undo-tree-visualizer-quit))))
+
+
+
+(defun undo-tree-draw-tree (undo-tree)
+  ;; Draw undo-tree in current buffer starting from NODE (or root if nil).
+  (let ((node (if undo-tree-visualizer-lazy-drawing
+		  (undo-tree-current undo-tree)
+		(undo-tree-root undo-tree))))
+    (erase-buffer)
+    (setq undo-tree-visualizer-needs-extending-down nil
+	  undo-tree-visualizer-needs-extending-up nil)
+    (undo-tree-clear-visualizer-data undo-tree)
+    (undo-tree-compute-widths node)
+    ;; lazy drawing starts vertically centred and displaced horizontally to
+    ;; the left (window-width/4), since trees will typically grow right
+    (if undo-tree-visualizer-lazy-drawing
+	(progn
+	  (undo-tree-move-down (/ (window-height) 2))
+	  (undo-tree-move-forward (max 2 (/ (window-width) 4)))) ; left margin
+      ;; non-lazy drawing starts in centre at top of buffer
+      (undo-tree-move-down 1)  ; top margin
+      (undo-tree-move-forward
+       (max (/ (window-width) 2)
+	    (+ (undo-tree-node-char-lwidth node)
+	       ;; add space for left part of left-most time-stamp
+	       (if undo-tree-visualizer-timestamps
+		   (/ (- undo-tree-visualizer-spacing 4) 2)
+		 0)
+	       2))))  ; left margin
+    ;; link starting node to its representation in visualizer
+    (setf (undo-tree-node-marker node) (make-marker))
+    (set-marker-insertion-type (undo-tree-node-marker node) nil)
+    (move-marker (undo-tree-node-marker node) (point))
+    ;; draw undo-tree
+    (let ((undo-tree-insert-face 'undo-tree-visualizer-default-face)
+	  node-list)
+      (if (not undo-tree-visualizer-lazy-drawing)
+	  (undo-tree-extend-down node t)
+	(undo-tree-extend-down node)
+	(undo-tree-extend-up node)
+	(setq node-list undo-tree-visualizer-needs-extending-down
+	      undo-tree-visualizer-needs-extending-down nil)
+	(while node-list (undo-tree-extend-down (pop node-list)))))
+    ;; highlight active branch
+    (let ((undo-tree-insert-face 'undo-tree-visualizer-active-branch-face))
+      (undo-tree-highlight-active-branch
+       (or undo-tree-visualizer-needs-extending-up
+	   (undo-tree-root undo-tree))))
+    ;; highlight current node
+    (undo-tree-draw-node (undo-tree-current undo-tree) 'current)))
+
+
+(defun undo-tree-extend-down (node &optional bottom)
+  ;; Extend tree downwards starting from NODE and point. If BOTTOM is t,
+  ;; extend all the way down to the leaves. If BOTTOM is a node, extend down
+  ;; as far as that node. If BOTTOM is an integer, extend down as far as that
+  ;; line. Otherwise, only extend visible portion of tree. NODE is assumed to
+  ;; already have a node marker. Returns non-nil if anything was actually
+  ;; extended.
+  (let ((extended nil)
+	(cur-stack (list node))
+	next-stack)
+    ;; don't bother extending if BOTTOM specifies an already-drawn node
+    (unless (and (undo-tree-node-p bottom) (undo-tree-node-marker bottom))
+      ;; draw nodes layer by layer
+      (while (or cur-stack
+		 (prog1 (setq cur-stack next-stack)
+		   (setq next-stack nil)))
+	(setq node (pop cur-stack))
+	;; if node is within range being drawn...
+	(if (or (eq bottom t)
+		(and (undo-tree-node-p bottom)
+		     (not (eq (undo-tree-node-previous node) bottom)))
+		(and (integerp bottom)
+		     (>= bottom (line-number-at-pos
+				 (undo-tree-node-marker node))))
+		(and (null bottom)
+		     (pos-visible-in-window-p (undo-tree-node-marker node)
+					      nil t)))
+	    ;; ...draw one layer of node's subtree (if not already drawn)
+	    (progn
+	      (unless (and (undo-tree-node-next node)
+			   (undo-tree-node-marker
+			    (nth (undo-tree-node-branch node)
+				 (undo-tree-node-next node))))
+		(goto-char (undo-tree-node-marker node))
+		(undo-tree-draw-subtree node)
+		(setq extended t))
+	      (setq next-stack
+		    (append (undo-tree-node-next node) next-stack)))
+	  ;; ...otherwise, postpone drawing until later
+	  (push node undo-tree-visualizer-needs-extending-down))))
+    extended))
+
+
+(defun undo-tree-extend-up (node &optional top)
+  ;; Extend tree upwards starting from NODE. If TOP is t, extend all the way
+  ;; to root. If TOP is a node, extend up as far as that node. If TOP is an
+  ;; integer, extend up as far as that line. Otherwise, only extend visible
+  ;; portion of tree. NODE is assumed to already have a node marker. Returns
+  ;; non-nil if anything was actually extended.
+  (let ((extended nil) parent)
+    ;; don't bother extending if TOP specifies an already-drawn node
+    (unless (and (undo-tree-node-p top) (undo-tree-node-marker top))
+      (while node
+	(setq parent (undo-tree-node-previous node))
+	;; if we haven't reached root...
+	(if parent
+	    ;; ...and node is within range being drawn...
+	    (if (or (eq top t)
+		    (and (undo-tree-node-p top) (not (eq node top)))
+		    (and (integerp top)
+			 (< top (line-number-at-pos
+				 (undo-tree-node-marker node))))
+		    (and (null top)
+			 ;; NOTE: we check point in case window-start is outdated
+			 (< (min (line-number-at-pos (point))
+				 (line-number-at-pos (window-start)))
+			    (line-number-at-pos
+			     (undo-tree-node-marker node)))))
+		;; ...and it hasn't already been drawn
+		(when (not (undo-tree-node-marker parent))
+		  ;; link parent node to its representation in visualizer
+		  (undo-tree-compute-widths parent)
+		  (undo-tree-move-to-parent node)
+		  (setf (undo-tree-node-marker parent) (make-marker))
+		  (set-marker-insertion-type
+		   (undo-tree-node-marker parent) nil)
+		  (move-marker (undo-tree-node-marker parent) (point))
+		  ;; draw subtree beneath parent
+		  (setq undo-tree-visualizer-needs-extending-down
+			(nconc (delq node (undo-tree-draw-subtree parent))
+			       undo-tree-visualizer-needs-extending-down))
+		  (setq extended t))
+	      ;; ...otherwise, postpone drawing for later and exit
+	      (setq undo-tree-visualizer-needs-extending-up (when parent node)
+		    parent nil))
+
+	  ;; if we've reached root, stop extending and add top margin
+	  (setq undo-tree-visualizer-needs-extending-up nil)
+	  (goto-char (undo-tree-node-marker node))
+	  (undo-tree-move-up 1)  ; top margin
+	  (delete-region (point-min) (line-beginning-position)))
+	;; next iteration
+	(setq node parent)))
+    extended))
+
+
+(defun undo-tree-expand-down (from &optional to)
+  ;; Expand tree downwards. FROM is the node to start expanding from. Stop
+  ;; expanding at TO if specified. Otherwise, just expand visible portion of
+  ;; tree and highlight active branch from FROM.
+  (when undo-tree-visualizer-needs-extending-down
+    (let ((inhibit-read-only t)
+	  node-list extended)
+      ;; extend down as far as TO node
+      (when to
+	(setq extended (undo-tree-extend-down from to))
+	(goto-char (undo-tree-node-marker to))
+	(redisplay t))  ; force redisplay to scroll buffer if necessary
+      ;; extend visible portion of tree downwards
+      (setq node-list undo-tree-visualizer-needs-extending-down
+	    undo-tree-visualizer-needs-extending-down nil)
+      (when node-list
+	(dolist (n node-list)
+	  (when (undo-tree-extend-down n) (setq extended t)))
+	;; highlight active branch in newly-extended-down portion, if any
+	(when extended
+	  (let ((undo-tree-insert-face
+		 'undo-tree-visualizer-active-branch-face))
+	    (undo-tree-highlight-active-branch from)))))))
+
+
+(defun undo-tree-expand-up (from &optional to)
+  ;; Expand tree upwards. FROM is the node to start expanding from, TO is the
+  ;; node to stop expanding at. If TO node isn't specified, just expand visible
+  ;; portion of tree and highlight active branch down to FROM.
+  (when undo-tree-visualizer-needs-extending-up
+    (let ((inhibit-read-only t)
+	  extended node-list)
+      ;; extend up as far as TO node
+      (when to
+	(setq extended (undo-tree-extend-up from to))
+	(goto-char (undo-tree-node-marker to))
+	;; simulate auto-scrolling if close to top of buffer
+	(when (<= (line-number-at-pos (point)) scroll-margin)
+	  (undo-tree-move-up (if (= scroll-conservatively 0)
+				 (/ (window-height) 2) 3))
+	  (when (undo-tree-extend-up to) (setq extended t))
+	  (goto-char (undo-tree-node-marker to))
+	  (unless (= scroll-conservatively 0) (recenter scroll-margin))))
+      ;; extend visible portion of tree upwards
+      (and undo-tree-visualizer-needs-extending-up
+	   (undo-tree-extend-up undo-tree-visualizer-needs-extending-up)
+	   (setq extended t))
+      ;; extend visible portion of tree downwards
+      (setq node-list undo-tree-visualizer-needs-extending-down
+	    undo-tree-visualizer-needs-extending-down nil)
+      (dolist (n node-list) (undo-tree-extend-down n))
+      ;; highlight active branch in newly-extended-up portion, if any
+      (when extended
+	(let ((undo-tree-insert-face
+	       'undo-tree-visualizer-active-branch-face))
+	  (undo-tree-highlight-active-branch
+	   (or undo-tree-visualizer-needs-extending-up
+	       (undo-tree-root buffer-undo-tree))
+	   from))))))
+
+
+
+(defun undo-tree-highlight-active-branch (node &optional end)
+  ;; Draw highlighted active branch below NODE in current buffer. Stop
+  ;; highlighting at END node if specified.
+  (let ((stack (list node)))
+    ;; draw active branch
+    (while stack
+      (setq node (pop stack))
+      (unless (or (eq node end)
+		  (memq node undo-tree-visualizer-needs-extending-down))
+	(goto-char (undo-tree-node-marker node))
+	(setq node (undo-tree-draw-subtree node 'active)
+	      stack (nconc stack node))))))
+
+
+(defun undo-tree-draw-node (node &optional current)
+  ;; Draw symbol representing NODE in visualizer. If CURRENT is non-nil, node
+  ;; is current node.
+  (goto-char (undo-tree-node-marker node))
+  (when undo-tree-visualizer-timestamps
+    (undo-tree-move-backward (/ undo-tree-visualizer-spacing 2)))
+
+  (let* ((undo-tree-insert-face (and undo-tree-insert-face
+				     (or (and (consp undo-tree-insert-face)
+					      undo-tree-insert-face)
+					 (list undo-tree-insert-face))))
+	 (register (undo-tree-node-register node))
+	 (unmodified (if undo-tree-visualizer-parent-mtime
+			 (undo-tree-node-unmodified-p
+			  node undo-tree-visualizer-parent-mtime)
+		       (undo-tree-node-unmodified-p node)))
+	node-string)
+    ;; check node's register (if any) still stores appropriate undo-tree state
+    (unless (and register
+		 (undo-tree-register-data-p
+		  (registerv-data (get-register register)))
+		 (eq node (undo-tree-register-data-node
+			   (registerv-data (get-register register)))))
+      (setq register nil))
+    ;; represent node by different symbols, depending on whether it's the
+    ;; current node, is saved in a register, or corresponds to an unmodified
+    ;; buffer
+    (setq node-string
+	    (cond
+	     (undo-tree-visualizer-timestamps
+	        (undo-tree-timestamp-to-string
+	         (undo-tree-node-timestamp node)
+		 undo-tree-visualizer-relative-timestamps
+		 current register))
+	     (register (char-to-string register))
+	     (unmodified "s")
+	     (current "x")
+	     (t "o"))
+	  undo-tree-insert-face
+	    (nconc
+	     (cond
+	      (current    '(undo-tree-visualizer-current-face))
+	      (unmodified '(undo-tree-visualizer-unmodified-face))
+	      (register   '(undo-tree-visualizer-register-face)))
+	     undo-tree-insert-face))
+    ;; draw node and link it to its representation in visualizer
+    (undo-tree-insert node-string)
+    (undo-tree-move-backward (if undo-tree-visualizer-timestamps
+				 (1+ (/ undo-tree-visualizer-spacing 2))
+			       1))
+    (move-marker (undo-tree-node-marker node) (point))
+    (put-text-property (point) (1+ (point)) 'undo-tree-node node)))
+
+
+(defun undo-tree-draw-subtree (node &optional active-branch)
+  ;; Draw subtree rooted at NODE. The subtree will start from point.
+  ;; If ACTIVE-BRANCH is non-nil, just draw active branch below NODE. Returns
+  ;; list of nodes below NODE.
+  (let ((num-children (length (undo-tree-node-next node)))
+        node-list pos trunk-pos n)
+    ;; draw node itself
+    (undo-tree-draw-node node)
+
+    (cond
+     ;; if we're at a leaf node, we're done
+     ((= num-children 0))
+
+     ;; if node has only one child, draw it (not strictly necessary to deal
+     ;; with this case separately, but as it's by far the most common case
+     ;; this makes the code clearer and more efficient)
+     ((= num-children 1)
+      (undo-tree-move-down 1)
+      (undo-tree-insert ?|)
+      (undo-tree-move-backward 1)
+      (undo-tree-move-down 1)
+      (undo-tree-insert ?|)
+      (undo-tree-move-backward 1)
+      (undo-tree-move-down 1)
+      (setq n (car (undo-tree-node-next node)))
+      ;; link next node to its representation in visualizer
+      (unless (markerp (undo-tree-node-marker n))
+        (setf (undo-tree-node-marker n) (make-marker))
+        (set-marker-insertion-type (undo-tree-node-marker n) nil))
+      (move-marker (undo-tree-node-marker n) (point))
+      ;; add next node to list of nodes to draw next
+      (push n node-list))
+
+     ;; if node has multiple children, draw branches
+     (t
+      (undo-tree-move-down 1)
+      (undo-tree-insert ?|)
+      (undo-tree-move-backward 1)
+      (move-marker (setq trunk-pos (make-marker)) (point))
+      ;; left subtrees
+      (undo-tree-move-backward
+       (- (undo-tree-node-char-lwidth node)
+          (undo-tree-node-char-lwidth
+           (car (undo-tree-node-next node)))))
+      (move-marker (setq pos (make-marker)) (point))
+      (setq n (cons nil (undo-tree-node-next node)))
+      (dotimes (i (/ num-children 2))
+        (setq n (cdr n))
+        (when (or (null active-branch)
+                  (eq (car n)
+                      (nth (undo-tree-node-branch node)
+                           (undo-tree-node-next node))))
+          (undo-tree-move-forward 2)
+          (undo-tree-insert ?_ (- trunk-pos pos 2))
+          (goto-char pos)
+          (undo-tree-move-forward 1)
+          (undo-tree-move-down 1)
+          (undo-tree-insert ?/)
+          (undo-tree-move-backward 2)
+          (undo-tree-move-down 1)
+          ;; link node to its representation in visualizer
+          (unless (markerp (undo-tree-node-marker (car n)))
+            (setf (undo-tree-node-marker (car n)) (make-marker))
+            (set-marker-insertion-type (undo-tree-node-marker (car n)) nil))
+          (move-marker (undo-tree-node-marker (car n)) (point))
+          ;; add node to list of nodes to draw next
+          (push (car n) node-list))
+        (goto-char pos)
+        (undo-tree-move-forward
+         (+ (undo-tree-node-char-rwidth (car n))
+            (undo-tree-node-char-lwidth (cadr n))
+            undo-tree-visualizer-spacing 1))
+        (move-marker pos (point)))
+      ;; middle subtree (only when number of children is odd)
+      (when (= (mod num-children 2) 1)
+        (setq n (cdr n))
+        (when (or (null active-branch)
+                  (eq (car n)
+                      (nth (undo-tree-node-branch node)
+                           (undo-tree-node-next node))))
+          (undo-tree-move-down 1)
+          (undo-tree-insert ?|)
+          (undo-tree-move-backward 1)
+          (undo-tree-move-down 1)
+          ;; link node to its representation in visualizer
+          (unless (markerp (undo-tree-node-marker (car n)))
+            (setf (undo-tree-node-marker (car n)) (make-marker))
+            (set-marker-insertion-type (undo-tree-node-marker (car n)) nil))
+          (move-marker (undo-tree-node-marker (car n)) (point))
+          ;; add node to list of nodes to draw next
+          (push (car n) node-list))
+        (goto-char pos)
+        (undo-tree-move-forward
+         (+ (undo-tree-node-char-rwidth (car n))
+            (if (cadr n) (undo-tree-node-char-lwidth (cadr n)) 0)
+            undo-tree-visualizer-spacing 1))
+        (move-marker pos (point)))
+      ;; right subtrees
+      (move-marker trunk-pos (1+ trunk-pos))
+      (dotimes (i (/ num-children 2))
+        (setq n (cdr n))
+        (when (or (null active-branch)
+                  (eq (car n)
+                      (nth (undo-tree-node-branch node)
+                           (undo-tree-node-next node))))
+          (goto-char trunk-pos)
+          (undo-tree-insert ?_ (- pos trunk-pos 1))
+          (goto-char pos)
+          (undo-tree-move-backward 1)
+          (undo-tree-move-down 1)
+          (undo-tree-insert ?\\)
+          (undo-tree-move-down 1)
+          ;; link node to its representation in visualizer
+          (unless (markerp (undo-tree-node-marker (car n)))
+            (setf (undo-tree-node-marker (car n)) (make-marker))
+            (set-marker-insertion-type (undo-tree-node-marker (car n)) nil))
+          (move-marker (undo-tree-node-marker (car n)) (point))
+          ;; add node to list of nodes to draw next
+          (push (car n) node-list))
+        (when (cdr n)
+          (goto-char pos)
+          (undo-tree-move-forward
+           (+ (undo-tree-node-char-rwidth (car n))
+              (if (cadr n) (undo-tree-node-char-lwidth (cadr n)) 0)
+              undo-tree-visualizer-spacing 1))
+          (move-marker pos (point))))
+      ))
+    ;; return list of nodes to draw next
+    (nreverse node-list)))
+
+
+(defun undo-tree-node-char-lwidth (node)
+  ;; Return left-width of NODE measured in characters.
+  (if (= (length (undo-tree-node-next node)) 0) 0
+    (- (* (+ undo-tree-visualizer-spacing 1) (undo-tree-node-lwidth node))
+       (if (= (undo-tree-node-cwidth node) 0)
+           (1+ (/ undo-tree-visualizer-spacing 2)) 0))))
+
+
+(defun undo-tree-node-char-rwidth (node)
+  ;; Return right-width of NODE measured in characters.
+  (if (= (length (undo-tree-node-next node)) 0) 0
+    (- (* (+ undo-tree-visualizer-spacing 1) (undo-tree-node-rwidth node))
+       (if (= (undo-tree-node-cwidth node) 0)
+           (1+ (/ undo-tree-visualizer-spacing 2)) 0))))
+
+
+(defun undo-tree-insert (str &optional arg)
+  ;; Insert character or string STR ARG times, overwriting, and using
+  ;; `undo-tree-insert-face'.
+  (unless arg (setq arg 1))
+  (when (characterp str)
+    (setq str (make-string arg str))
+    (setq arg 1))
+  (dotimes (i arg) (insert str))
+  (setq arg (* arg (length str)))
+  (undo-tree-move-forward arg)
+  ;; make sure mark isn't active, otherwise `backward-delete-char' might
+  ;; delete region instead of single char if transient-mark-mode is enabled
+  (setq mark-active nil)
+  (backward-delete-char arg)
+  (when undo-tree-insert-face
+    (put-text-property (- (point) arg) (point) 'face undo-tree-insert-face)))
+
+
+(defun undo-tree-move-down (&optional arg)
+  ;; Move down, extending buffer if necessary.
+  (let ((row (line-number-at-pos))
+        (col (current-column))
+        line)
+    (unless arg (setq arg 1))
+    (forward-line arg)
+    (setq line (line-number-at-pos))
+    ;; if buffer doesn't have enough lines, add some
+    (when (/= line (+ row arg))
+      (cond
+       ((< arg 0)
+	(insert (make-string (- line row arg) ?\n))
+	(forward-line (+ arg (- row line))))
+       (t (insert (make-string (- arg (- line row)) ?\n)))))
+    (undo-tree-move-forward col)))
+
+
+(defun undo-tree-move-up (&optional arg)
+  ;; Move up, extending buffer if necessary.
+  (unless arg (setq arg 1))
+  (undo-tree-move-down (- arg)))
+
+
+(defun undo-tree-move-forward (&optional arg)
+  ;; Move forward, extending buffer if necessary.
+  (unless arg (setq arg 1))
+  (let (n)
+    (cond
+     ((>= arg 0)
+      (setq n (- (line-end-position) (point)))
+      (if (> n arg)
+	  (forward-char arg)
+	(end-of-line)
+	(insert (make-string (- arg n) ? ))))
+     ((< arg 0)
+      (setq arg (- arg))
+      (setq n (- (point) (line-beginning-position)))
+      (when (< (- n 2) arg)  ; -2 to create left-margin
+	;; no space left - shift entire buffer contents right!
+	(let ((pos (move-marker (make-marker) (point))))
+	  (set-marker-insertion-type pos t)
+	  (goto-char (point-min))
+	  (while (not (eobp))
+	    (insert-before-markers (make-string (- arg -2 n) ? ))
+	    (forward-line 1))
+	  (goto-char pos)))
+      (backward-char arg)))))
+
+
+(defun undo-tree-move-backward (&optional arg)
+  ;; Move backward, extending buffer if necessary.
+  (unless arg (setq arg 1))
+  (undo-tree-move-forward (- arg)))
+
+
+(defun undo-tree-move-to-parent (node)
+  ;; Move to position of parent of NODE, extending buffer if necessary.
+  (let* ((parent (undo-tree-node-previous node))
+	 (n (undo-tree-node-next parent))
+	 (l (length n)) p)
+    (goto-char (undo-tree-node-marker node))
+    (unless (= l 1)
+      ;; move horizontally
+      (setq p (undo-tree-position node n))
+      (cond
+       ;; node in centre subtree: no horizontal movement
+       ((and (= (mod l 2) 1) (= p (/ l 2))))
+       ;; node in left subtree: move right
+       ((< p (/ l 2))
+	(setq n (nthcdr p n))
+	(undo-tree-move-forward
+	 (+ (undo-tree-node-char-rwidth (car n))
+	    (/ undo-tree-visualizer-spacing 2) 1))
+	(dotimes (i (- (/ l 2) p 1))
+	  (setq n (cdr n))
+	  (undo-tree-move-forward
+	   (+ (undo-tree-node-char-lwidth (car n))
+	      (undo-tree-node-char-rwidth (car n))
+	      undo-tree-visualizer-spacing 1)))
+	(when (= (mod l 2) 1)
+	  (setq n (cdr n))
+	  (undo-tree-move-forward
+	   (+ (undo-tree-node-char-lwidth (car n))
+	      (/ undo-tree-visualizer-spacing 2) 1))))
+       (t ;; node in right subtree: move left
+	(setq n (nthcdr (/ l 2) n))
+	(when (= (mod l 2) 1)
+	  (undo-tree-move-backward
+	   (+ (undo-tree-node-char-rwidth (car n))
+	      (/ undo-tree-visualizer-spacing 2) 1))
+	  (setq n (cdr n)))
+	(dotimes (i (- p (/ l 2) (mod l 2)))
+	  (undo-tree-move-backward
+	   (+ (undo-tree-node-char-lwidth (car n))
+	      (undo-tree-node-char-rwidth (car n))
+	      undo-tree-visualizer-spacing 1))
+	  (setq n (cdr n)))
+	(undo-tree-move-backward
+	 (+ (undo-tree-node-char-lwidth (car n))
+	    (/ undo-tree-visualizer-spacing 2) 1)))))
+    ;; move vertically
+    (undo-tree-move-up 3)))
+
+
+(defun undo-tree-timestamp-to-string
+  (timestamp &optional relative current register)
+  ;; Convert TIMESTAMP to string (either absolute or RELATVE time), indicating
+  ;; if it's the CURRENT node and/or has an associated REGISTER.
+  (if relative
+      ;; relative time
+      (let ((time (floor (float-time
+			  (subtract-time (current-time) timestamp))))
+	    n)
+	(setq time
+	      ;; years
+	      (if (> (setq n (/ time 315360000)) 0)
+		  (if (> n 999) "-ages" (format "-%dy" n))
+		(setq time (% time 315360000))
+		;; days
+		(if (> (setq n (/ time 86400)) 0)
+		    (format "-%dd" n)
+		  (setq time (% time 86400))
+		  ;; hours
+		  (if (> (setq n (/ time 3600)) 0)
+		      (format "-%dh" n)
+		    (setq time (% time 3600))
+		    ;; mins
+		    (if (> (setq n (/ time 60)) 0)
+			(format "-%dm" n)
+		      ;; secs
+		      (format "-%ds" (% time 60)))))))
+	(setq time (concat
+		    (if current "*" " ")
+		    time
+		    (if register (concat "[" (char-to-string register) "]")
+		      "   ")))
+	(setq n (length time))
+	(if (< n 9)
+	    (concat (make-string (- 9 n) ? ) time)
+	  time))
+    ;; absolute time
+    (concat (if current " *" "  ")
+	    (format-time-string "%H:%M:%S" timestamp)
+	    (if register
+		(concat "[" (char-to-string register) "]")
+	      "   "))))
+
+
+
+
+;;; =====================================================================
+;;;                        Visualizer commands
+
+(define-derived-mode
+  undo-tree-visualizer-mode special-mode "undo-tree-visualizer"
+  "Major mode used in undo-tree visualizer.
+
+The undo-tree visualizer can only be invoked from a buffer in
+which `undo-tree-mode' is enabled. The visualizer displays the
+undo history tree graphically, and allows you to browse around
+the undo history, undoing or redoing the corresponding changes in
+the parent buffer.
+
+Within the undo-tree visualizer, the following keys are available:
+
+  \\{undo-tree-visualizer-mode-map}"
+  :syntax-table nil
+  :abbrev-table nil
+  (setq truncate-lines t)
+  (setq cursor-type nil)
+  (setq undo-tree-visualizer-selected-node nil))
+
+
+
+(defun undo-tree-visualize-undo (&optional arg)
+  "Undo changes. A numeric ARG serves as a repeat count."
+  (interactive "p")
+  (let ((old (undo-tree-current buffer-undo-tree))
+	current)
+    ;; unhighlight old current node
+    (let ((undo-tree-insert-face 'undo-tree-visualizer-active-branch-face)
+	  (inhibit-read-only t))
+      (undo-tree-draw-node old))
+    ;; undo in parent buffer
+    (switch-to-buffer-other-window undo-tree-visualizer-parent-buffer)
+    (deactivate-mark)
+    (unwind-protect
+	(let ((undo-tree-inhibit-kill-visualizer t)) (undo-tree-undo-1 arg))
+      (setq current (undo-tree-current buffer-undo-tree))
+      (switch-to-buffer-other-window undo-tree-visualizer-buffer-name)
+      ;; when using lazy drawing, extend tree upwards as required
+      (when undo-tree-visualizer-lazy-drawing
+	(undo-tree-expand-up old current))
+      ;; highlight new current node
+      (let ((inhibit-read-only t)) (undo-tree-draw-node current 'current))
+      ;; update diff display, if any
+      (when undo-tree-visualizer-diff (undo-tree-visualizer-update-diff)))))
+
+
+(defun undo-tree-visualize-redo (&optional arg)
+  "Redo changes. A numeric ARG serves as a repeat count."
+  (interactive "p")
+  (let ((old (undo-tree-current buffer-undo-tree))
+	current)
+    ;; unhighlight old current node
+    (let ((undo-tree-insert-face 'undo-tree-visualizer-active-branch-face)
+	  (inhibit-read-only t))
+      (undo-tree-draw-node (undo-tree-current buffer-undo-tree)))
+    ;; redo in parent buffer
+    (switch-to-buffer-other-window undo-tree-visualizer-parent-buffer)
+    (deactivate-mark)
+    (unwind-protect
+	(let ((undo-tree-inhibit-kill-visualizer t)) (undo-tree-redo-1 arg))
+      (setq current (undo-tree-current buffer-undo-tree))
+      (switch-to-buffer-other-window undo-tree-visualizer-buffer-name)
+      ;; when using lazy drawing, extend tree downwards as required
+      (when undo-tree-visualizer-lazy-drawing
+	(undo-tree-expand-down old current))
+      ;; highlight new current node
+      (let ((inhibit-read-only t)) (undo-tree-draw-node current 'current))
+      ;; update diff display, if any
+      (when undo-tree-visualizer-diff (undo-tree-visualizer-update-diff)))))
+
+
+(defun undo-tree-visualize-switch-branch-right (arg)
+  "Switch to next branch of the undo tree.
+This will affect which branch to descend when *redoing* changes
+using `undo-tree-redo' or `undo-tree-visualizer-redo'."
+  (interactive "p")
+  ;; un-highlight old active branch below current node
+  (goto-char (undo-tree-node-marker (undo-tree-current buffer-undo-tree)))
+  (let ((undo-tree-insert-face 'undo-tree-visualizer-default-face)
+	(inhibit-read-only t))
+    (undo-tree-highlight-active-branch (undo-tree-current buffer-undo-tree)))
+  ;; increment branch
+  (let ((branch (undo-tree-node-branch (undo-tree-current buffer-undo-tree))))
+  (setf (undo-tree-node-branch (undo-tree-current buffer-undo-tree))
+        (cond
+         ((>= (+ branch arg) (undo-tree-num-branches))
+          (1- (undo-tree-num-branches)))
+         ((<= (+ branch arg) 0) 0)
+         (t (+ branch arg))))
+  (let ((inhibit-read-only t))
+    ;; highlight new active branch below current node
+    (goto-char (undo-tree-node-marker (undo-tree-current buffer-undo-tree)))
+    (let ((undo-tree-insert-face 'undo-tree-visualizer-active-branch-face))
+      (undo-tree-highlight-active-branch (undo-tree-current buffer-undo-tree)))
+    ;; re-highlight current node
+    (undo-tree-draw-node (undo-tree-current buffer-undo-tree) 'current))))
+
+
+(defun undo-tree-visualize-switch-branch-left (arg)
+  "Switch to previous branch of the undo tree.
+This will affect which branch to descend when *redoing* changes
+using `undo-tree-redo' or `undo-tree-visualizer-redo'."
+  (interactive "p")
+  (undo-tree-visualize-switch-branch-right (- arg)))
+
+
+(defun undo-tree-visualizer-quit ()
+  "Quit the undo-tree visualizer."
+  (interactive)
+  (undo-tree-clear-visualizer-data buffer-undo-tree)
+  ;; remove kill visualizer hook from parent buffer
+  (unwind-protect
+      (with-current-buffer undo-tree-visualizer-parent-buffer
+	(remove-hook 'before-change-functions 'undo-tree-kill-visualizer t))
+    ;; kill diff buffer, if any
+    (when undo-tree-visualizer-diff (undo-tree-visualizer-hide-diff))
+    (let ((parent undo-tree-visualizer-parent-buffer)
+	  window)
+      ;; kill visualizer buffer
+      (kill-buffer nil)
+      ;; switch back to parent buffer
+      (unwind-protect
+	  (if (setq window (get-buffer-window parent))
+	      (select-window window)
+	    (switch-to-buffer parent))))))
+
+
+(defun undo-tree-visualizer-abort ()
+  "Quit the undo-tree visualizer and return buffer to original state."
+  (interactive)
+  (let ((node undo-tree-visualizer-initial-node))
+    (undo-tree-visualizer-quit)
+    (undo-tree-set node)))
+
+
+(defun undo-tree-visualizer-set (&optional pos)
+  "Set buffer to state corresponding to undo tree node
+at POS, or point if POS is nil."
+  (interactive)
+  (unless pos (setq pos (point)))
+  (let ((node (get-text-property pos 'undo-tree-node)))
+    (when node
+      ;; set parent buffer to state corresponding to node at POS
+      (switch-to-buffer-other-window undo-tree-visualizer-parent-buffer)
+      (let ((undo-tree-inhibit-kill-visualizer t)) (undo-tree-set node))
+      (switch-to-buffer-other-window undo-tree-visualizer-buffer-name)
+      ;; re-draw undo tree
+      (let ((inhibit-read-only t)) (undo-tree-draw-tree buffer-undo-tree))
+      (when undo-tree-visualizer-diff (undo-tree-visualizer-update-diff)))))
+
+
+(defun undo-tree-visualizer-mouse-set (pos)
+  "Set buffer to state corresponding to undo tree node
+at mouse event POS."
+  (interactive "@e")
+  (undo-tree-visualizer-set (event-start (nth 1 pos))))
+
+
+(defun undo-tree-visualize-undo-to-x (&optional x)
+  "Undo to last branch point, register, or saved state.
+If X is the symbol `branch', undo to last branch point. If X is
+the symbol `register', undo to last register. If X is the sumbol
+`saved', undo to last saved state. If X is null, undo to first of
+these that's encountered.
+
+Interactively, a single \\[universal-argument] specifies
+`branch', a double \\[universal-argument] \\[universal-argument]
+specifies `saved', and a negative prefix argument specifies
+`register'."
+  (interactive "P")
+  (when (and (called-interactively-p 'any) x)
+    (setq x (prefix-numeric-value x)
+	  x (cond
+	     ((< x 0)  'register)
+	     ((<= x 4) 'branch)
+	     (t        'saved))))
+  (let ((current (if undo-tree-visualizer-selection-mode
+		     undo-tree-visualizer-selected-node
+		   (undo-tree-current buffer-undo-tree)))
+	(diff undo-tree-visualizer-diff)
+	r)
+    (undo-tree-visualizer-hide-diff)
+    (while (and (undo-tree-node-previous current)
+		(or (if undo-tree-visualizer-selection-mode
+			(progn
+			  (undo-tree-visualizer-select-previous)
+			  (setq current undo-tree-visualizer-selected-node))
+		      (undo-tree-visualize-undo)
+		      (setq current (undo-tree-current buffer-undo-tree)))
+		    t)
+		         ;; branch point
+		(not (or (and (or (null x) (eq x 'branch))
+			      (> (undo-tree-num-branches) 1))
+			 ;; register
+			 (and (or (null x) (eq x 'register))
+			      (setq r (undo-tree-node-register current))
+			      (undo-tree-register-data-p
+			       (setq r (registerv-data (get-register r))))
+			      (eq current (undo-tree-register-data-node r)))
+			 ;; saved state
+			 (and (or (null x) (eq x 'saved))
+			      (undo-tree-node-unmodified-p current))
+			 ))))
+    ;; update diff display, if any
+    (when diff
+      (undo-tree-visualizer-show-diff
+       (when undo-tree-visualizer-selection-mode
+	 undo-tree-visualizer-selected-node)))))
+
+
+(defun undo-tree-visualize-redo-to-x (&optional x)
+  "Redo to last branch point, register, or saved state.
+If X is the symbol `branch', redo to last branch point. If X is
+the symbol `register', redo to last register. If X is the sumbol
+`saved', redo to last saved state. If X is null, redo to first of
+these that's encountered.
+
+Interactively, a single \\[universal-argument] specifies
+`branch', a double \\[universal-argument] \\[universal-argument]
+specifies `saved', and a negative prefix argument specifies
+`register'."
+  (interactive "P")
+  (when (and (called-interactively-p 'any) x)
+    (setq x (prefix-numeric-value x)
+	  x (cond
+	     ((< x 0)  'register)
+	     ((<= x 4) 'branch)
+	     (t        'saved))))
+  (let ((current (if undo-tree-visualizer-selection-mode
+		     undo-tree-visualizer-selected-node
+		   (undo-tree-current buffer-undo-tree)))
+	(diff undo-tree-visualizer-diff)
+	r)
+    (undo-tree-visualizer-hide-diff)
+    (while (and (undo-tree-node-next current)
+		(or (if undo-tree-visualizer-selection-mode
+			(progn
+			  (undo-tree-visualizer-select-next)
+			  (setq current undo-tree-visualizer-selected-node))
+		      (undo-tree-visualize-redo)
+		      (setq current (undo-tree-current buffer-undo-tree)))
+		    t)
+		         ;; branch point
+		(not (or (and (or (null x) (eq x 'branch))
+			      (> (undo-tree-num-branches) 1))
+			 ;; register
+			 (and (or (null x) (eq x 'register))
+			      (setq r (undo-tree-node-register current))
+			      (undo-tree-register-data-p
+			       (setq r (registerv-data (get-register r))))
+			      (eq current (undo-tree-register-data-node r)))
+			 ;; saved state
+			 (and (or (null x) (eq x 'saved))
+			      (undo-tree-node-unmodified-p current))
+			 ))))
+    ;; update diff display, if any
+    (when diff
+      (undo-tree-visualizer-show-diff
+       (when undo-tree-visualizer-selection-mode
+	 undo-tree-visualizer-selected-node)))))
+
+
+(defun undo-tree-visualizer-toggle-timestamps ()
+  "Toggle display of time-stamps."
+  (interactive)
+  (setq undo-tree-visualizer-timestamps (not undo-tree-visualizer-timestamps))
+  (setq undo-tree-visualizer-spacing (undo-tree-visualizer-calculate-spacing))
+  ;; redraw tree
+  (let ((inhibit-read-only t)) (undo-tree-draw-tree buffer-undo-tree)))
+
+
+(defun undo-tree-visualizer-scroll-left (&optional arg)
+  (interactive "p")
+  (scroll-left (or arg 1) t))
+
+
+(defun undo-tree-visualizer-scroll-right (&optional arg)
+  (interactive "p")
+  (scroll-right (or arg 1) t))
+
+
+(defun undo-tree-visualizer-scroll-up (&optional arg)
+  (interactive "P")
+  (if (or (and (numberp arg) (< arg 0)) (eq arg '-))
+      (undo-tree-visualizer-scroll-down arg)
+    ;; scroll up and expand newly-visible portion of tree
+    (unwind-protect
+	(scroll-up-command arg)
+      (undo-tree-expand-down
+       (nth (undo-tree-node-branch (undo-tree-current buffer-undo-tree))
+	    (undo-tree-node-next (undo-tree-current buffer-undo-tree)))))
+    ;; signal error if at eob
+    (when (and (not undo-tree-visualizer-needs-extending-down) (eobp))
+      (scroll-up))))
+
+
+(defun undo-tree-visualizer-scroll-down (&optional arg)
+  (interactive "P")
+  (if (or (and (numberp arg) (< arg 0)) (eq arg '-))
+      (undo-tree-visualizer-scroll-up arg)
+    ;; ensure there's enough room at top of buffer to scroll
+    (let ((scroll-lines
+	   (or arg (- (window-height) next-screen-context-lines)))
+	  (window-line (1- (line-number-at-pos (window-start)))))
+      (when (and undo-tree-visualizer-needs-extending-up
+		 (< window-line scroll-lines))
+	(let ((inhibit-read-only t))
+	  (goto-char (point-min))
+	  (undo-tree-move-up (- scroll-lines window-line)))))
+    ;; scroll down and expand newly-visible portion of tree
+    (unwind-protect
+	(scroll-down-command arg)
+      (undo-tree-expand-up
+       (undo-tree-node-previous (undo-tree-current buffer-undo-tree))))
+    ;; signal error if at bob
+    (when (and (not undo-tree-visualizer-needs-extending-down) (bobp))
+      (scroll-down))))
+
+
+
+
+;;; =====================================================================
+;;;                    Visualizer selection mode
+
+(define-minor-mode undo-tree-visualizer-selection-mode
+  "Toggle mode to select nodes in undo-tree visualizer."
+  :lighter "Select"
+  :keymap undo-tree-visualizer-selection-mode-map
+  :group undo-tree
+  (cond
+   ;; enable selection mode
+   (undo-tree-visualizer-selection-mode
+    (setq cursor-type 'box)
+    (setq undo-tree-visualizer-selected-node
+	  (undo-tree-current buffer-undo-tree))
+    ;; erase diff (if any), as initially selected node is identical to current
+    (when undo-tree-visualizer-diff
+      (let ((buff (get-buffer undo-tree-diff-buffer-name))
+	    (inhibit-read-only t))
+	(when buff (with-current-buffer buff (erase-buffer))))))
+   (t ;; disable selection mode
+    (setq cursor-type nil)
+    (setq undo-tree-visualizer-selected-node nil)
+    (goto-char (undo-tree-node-marker (undo-tree-current buffer-undo-tree)))
+    (when undo-tree-visualizer-diff (undo-tree-visualizer-update-diff)))
+   ))
+
+
+(defun undo-tree-visualizer-select-previous (&optional arg)
+  "Move to previous node."
+  (interactive "p")
+  (let ((node undo-tree-visualizer-selected-node))
+    (catch 'top
+      (dotimes (i (or arg 1))
+	(unless (undo-tree-node-previous node) (throw 'top t))
+	(setq node (undo-tree-node-previous node))))
+    ;; when using lazy drawing, extend tree upwards as required
+    (when undo-tree-visualizer-lazy-drawing
+      (undo-tree-expand-up undo-tree-visualizer-selected-node node))
+    ;; update diff display, if any
+    (when (and undo-tree-visualizer-diff
+	       (not (eq node undo-tree-visualizer-selected-node)))
+      (undo-tree-visualizer-update-diff node))
+    ;; move to selected node
+    (goto-char (undo-tree-node-marker node))
+    (setq undo-tree-visualizer-selected-node node)))
+
+
+(defun undo-tree-visualizer-select-next (&optional arg)
+  "Move to next node."
+  (interactive "p")
+  (let ((node undo-tree-visualizer-selected-node))
+    (catch 'bottom
+      (dotimes (i (or arg 1))
+	(unless (nth (undo-tree-node-branch node) (undo-tree-node-next node))
+	  (throw 'bottom t))
+	(setq node
+	      (nth (undo-tree-node-branch node) (undo-tree-node-next node)))))
+    ;; when using lazy drawing, extend tree downwards as required
+    (when undo-tree-visualizer-lazy-drawing
+      (undo-tree-expand-down undo-tree-visualizer-selected-node node))
+    ;; update diff display, if any
+    (when (and undo-tree-visualizer-diff
+	       (not (eq node undo-tree-visualizer-selected-node)))
+      (undo-tree-visualizer-update-diff node))
+    ;; move to selected node
+    (goto-char (undo-tree-node-marker node))
+    (setq undo-tree-visualizer-selected-node node)))
+
+
+(defun undo-tree-visualizer-select-right (&optional arg)
+  "Move right to a sibling node."
+  (interactive "p")
+  (let ((node undo-tree-visualizer-selected-node)
+	end)
+    (goto-char (undo-tree-node-marker undo-tree-visualizer-selected-node))
+    (setq end (line-end-position))
+    (catch 'end
+      (dotimes (i arg)
+	(while (or (null node) (eq node undo-tree-visualizer-selected-node))
+	  (forward-char)
+	  (setq node (get-text-property (point) 'undo-tree-node))
+	  (when (= (point) end) (throw 'end t)))))
+    (goto-char (undo-tree-node-marker
+		(or node undo-tree-visualizer-selected-node)))
+    (when (and undo-tree-visualizer-diff node
+	       (not (eq node undo-tree-visualizer-selected-node)))
+      (undo-tree-visualizer-update-diff node))
+    (when node (setq undo-tree-visualizer-selected-node node))))
+
+
+(defun undo-tree-visualizer-select-left (&optional arg)
+  "Move left to a sibling node."
+  (interactive "p")
+  (let ((node (get-text-property (point) 'undo-tree-node))
+	beg)
+    (goto-char (undo-tree-node-marker undo-tree-visualizer-selected-node))
+    (setq beg (line-beginning-position))
+    (catch 'beg
+      (dotimes (i arg)
+	(while (or (null node) (eq node undo-tree-visualizer-selected-node))
+	  (backward-char)
+	  (setq node (get-text-property (point) 'undo-tree-node))
+	  (when (= (point) beg) (throw 'beg t)))))
+    (goto-char (undo-tree-node-marker
+		(or node undo-tree-visualizer-selected-node)))
+    (when (and undo-tree-visualizer-diff node
+	       (not (eq node undo-tree-visualizer-selected-node)))
+      (undo-tree-visualizer-update-diff node))
+    (when node (setq undo-tree-visualizer-selected-node node))))
+
+
+(defun undo-tree-visualizer-select (pos)
+  (let ((node (get-text-property pos 'undo-tree-node)))
+    (when node
+      ;; select node at POS
+      (goto-char (undo-tree-node-marker node))
+      ;; when using lazy drawing, extend tree up and down as required
+      (when undo-tree-visualizer-lazy-drawing
+	(undo-tree-expand-up undo-tree-visualizer-selected-node node)
+	(undo-tree-expand-down undo-tree-visualizer-selected-node node))
+      ;; update diff display, if any
+      (when (and undo-tree-visualizer-diff
+		 (not (eq node undo-tree-visualizer-selected-node)))
+	(undo-tree-visualizer-update-diff node))
+      ;; update selected node
+      (setq undo-tree-visualizer-selected-node node)
+      )))
+
+
+(defun undo-tree-visualizer-mouse-select (pos)
+  "Select undo tree node at mouse event POS."
+  (interactive "@e")
+  (undo-tree-visualizer-select (event-start (nth 1 pos))))
+
+
+
+
+;;; =====================================================================
+;;;                      Visualizer diff display
+
+(defun undo-tree-visualizer-toggle-diff ()
+  "Toggle diff display in undo-tree visualizer."
+  (interactive)
+  (if undo-tree-visualizer-diff
+      (undo-tree-visualizer-hide-diff)
+    (undo-tree-visualizer-show-diff)))
+
+
+(defun undo-tree-visualizer-selection-toggle-diff ()
+  "Toggle diff display in undo-tree visualizer selection mode."
+  (interactive)
+  (if undo-tree-visualizer-diff
+      (undo-tree-visualizer-hide-diff)
+    (let ((node (get-text-property (point) 'undo-tree-node)))
+      (when node (undo-tree-visualizer-show-diff node)))))
+
+
+(defun undo-tree-visualizer-show-diff (&optional node)
+  ;; show visualizer diff display
+  (setq undo-tree-visualizer-diff t)
+  (let ((buff (with-current-buffer undo-tree-visualizer-parent-buffer
+		(undo-tree-diff node)))
+	(display-buffer-mark-dedicated 'soft)
+	win)
+    (setq win (split-window))
+    (set-window-buffer win buff)
+    (shrink-window-if-larger-than-buffer win)))
+
+
+(defun undo-tree-visualizer-hide-diff ()
+  ;; hide visualizer diff display
+  (setq undo-tree-visualizer-diff nil)
+  (let ((win (get-buffer-window undo-tree-diff-buffer-name)))
+    (when win (with-selected-window win (kill-buffer-and-window)))))
+
+
+(defun undo-tree-diff (&optional node)
+  ;; Create diff between NODE and current state (or previous state and current
+  ;; state, if NODE is null). Returns buffer containing diff.
+  (let (tmpfile buff)
+    ;; generate diff
+    (let ((undo-tree-inhibit-kill-visualizer t)
+	  (current (undo-tree-current buffer-undo-tree)))
+      (undo-tree-set (or node (undo-tree-node-previous current) current)
+		     'preserve-timestamps)
+      (setq tmpfile (diff-file-local-copy (current-buffer)))
+      (undo-tree-set current 'preserve-timestamps))
+    (setq buff (diff-no-select
+		tmpfile (current-buffer) nil 'noasync
+		(get-buffer-create undo-tree-diff-buffer-name)))
+    ;; delete process messages and useless headers from diff buffer
+    (let ((inhibit-read-only t))
+      (with-current-buffer buff
+	(goto-char (point-min))
+	(delete-region (point) (1+ (line-end-position 3)))
+	(goto-char (point-max))
+	(forward-line -2)
+	(delete-region (point) (point-max))
+	(setq cursor-type nil)
+	(setq buffer-read-only t)))
+    buff))
+
+
+(defun undo-tree-visualizer-update-diff (&optional node)
+  ;; update visualizer diff display to show diff between current state and
+  ;; NODE (or previous state, if NODE is null)
+  (with-current-buffer undo-tree-visualizer-parent-buffer
+    (undo-tree-diff node))
+  (let ((win (get-buffer-window undo-tree-diff-buffer-name)))
+    (when win
+      (balance-windows)
+      (shrink-window-if-larger-than-buffer win))))
+
+;;;; ChangeLog:
+
+;; 2013-12-28  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	* undo-tree: Update to version 0.6.5.
+;; 
+;; 2012-12-05  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	Update undo-tree to version 0.6.3
+;; 
+;; 	* undo-tree.el: Implement lazy tree drawing to significantly speed up 
+;; 	visualization of large trees + various more minor improvements.
+;; 
+;; 2012-09-25  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	Updated undo-tree package to version 0.5.5.
+;; 
+;; 	Small bug-fix to avoid hooks triggering an error when trying to save
+;; 	undo history in a buffer where undo is disabled.
+;; 
+;; 2012-09-11  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	Updated undo-tree package to version 0.5.4
+;; 
+;; 	Bug-fixes and improvements to persistent history storage.
+;; 
+;; 2012-07-18  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	Update undo-tree to version 0.5.3
+;; 
+;; 	* undo-tree.el: Cope gracefully with undo boundaries being deleted
+;; 	 (cf. bug#11774). Allow customization of directory to which undo
+;; 	history is
+;; 	 saved.
+;; 
+;; 2012-05-24  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	updated undo-tree package to version 0.5.2
+;; 
+;; 	* undo-tree.el: add diff view feature in undo-tree visualizer.
+;; 
+;; 2012-05-02  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	undo-tree.el: Update package to version 0.4
+;; 
+;; 2012-04-20  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	undo-tree.el: Update package to version 0.3.4
+;; 
+;; 	* undo-tree.el (undo-list-pop-changeset): fix pernicious bug causing
+;; 	undo history to be lost.
+;; 	(buffer-undo-tree): set permanent-local property.
+;; 	(undo-tree-enable-undo-in-region): add new customization option
+;; 	allowing undo-in-region to be disabled.
+;; 
+;; 2012-01-26  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	undo-tree.el: Fixed copyright attribution and Emacs status.
+;; 
+;; 2012-01-26  Toby S. Cubitt  <tsc25@cantab.net>
+;; 
+;; 	undo-tree.el: Update package to version 0.3.3
+;; 
+;; 2011-09-17  Stefan Monnier  <monnier@iro.umontreal.ca>
+;; 
+;; 	Add undo-tree.el
+;; 
+
+
+
+
+(provide 'undo-tree)
+
+;;; undo-tree.el ends here