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diff --git a/users/Profpatsch/exactSource.nix b/users/Profpatsch/exactSource.nix
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+{ ... }:
+# SPDX-License-Identifier: MIT
+# Created by Graham Christensen
+# version from https://github.com/grahamc/mayday/blob/c48f7583e622fe2e695a2a929de34679e5818816/exact-source.nix
+
+let
+  # Require that every path specified does exist.
+  #
+  # By default, Nix won't complain if you refer to a missing file
+  # if you don't actually use it:
+  #
+  #     nix-repl> ./bogus
+  #     /home/grahamc/playground/bogus
+  #
+  #     nix-repl> toString ./bogus
+  #     "/home/grahamc/playground/bogus"
+  #
+  # so in order for this interface to be *exact*, we must
+  # specifically require every provided path exists:
+  #
+  #     nix-repl> "${./bogus}"
+  #     error: getting attributes of path
+  #     '/home/grahamc/playground/bogus': No such file or
+  #     directory
+  requireAllPathsExist = paths:
+    let
+      validation = builtins.map (path: "${path}") paths;
+    in
+    builtins.deepSeq validation paths;
+
+  # Break down a given path in to a list of all of the path and
+  # its parent directories.
+  #
+  # `builtins.path` / `builtins.filterSource` will ask about
+  # a containing directory, and we must say YES otherwise it will
+  # not include anything below it.
+  #
+  # Concretely, convert: "/foo/baz/tux" in to:
+  #     [ "/foo/baz/tux" "/foo/baz" "/foo" ]
+  recursivelyPopDir = path:
+    if path == "/" then [ ]
+    else [ path ] ++ (recursivelyPopDir (builtins.dirOf path));
+
+  # Given a list of of strings, dedup the list and return a
+  # list of all unique strings.
+  #
+  # Note: only works on strings ;):
+  #
+  # First convert [ "foo" "foo" "bar" ] in to:
+  #     [
+  #       { name = "foo"; value = ""; }
+  #       { name = "foo"; value = ""; }
+  #       { name = "bar"; value = ""; }
+  #     ]
+  # then convert that to { "foo" = ""; "bar" = ""; }
+  # then get the attribute names, "foo" and "bar".
+  dedup = strings:
+    let
+      name_value_pairs = builtins.map
+        (string: { name = string; value = ""; })
+        strings;
+      attrset_of_strings = builtins.listToAttrs name_value_pairs;
+    in
+    builtins.attrNames attrset_of_strings;
+
+  exactSource = source_root: paths:
+    let
+      all_possible_paths =
+        let
+          # Convert all the paths in to relative paths on disk.
+          # ie: stringPaths will contain [ "/home/grahamc/playground/..." ];
+          # instead of /nix/store paths.
+          string_paths = builtins.map toString
+            (requireAllPathsExist paths);
+
+          all_paths_with_duplicates = builtins.concatMap
+            recursivelyPopDir
+            string_paths;
+        in
+        dedup all_paths_with_duplicates;
+
+      pathIsSpecified = path:
+        builtins.elem path all_possible_paths;
+    in
+    builtins.path {
+      path = source_root;
+      filter = (path: _type: pathIsSpecified path);
+    };
+in
+exactSource