* Allow conflicting packages to be kept in a user environment, and

  allow switching between them (NIX-80).

  Example: two versions of Pan:

    $ nix-env -q pan
    pan-0.128
    pan-0.14.2.91

    $ readlink $(which pan)
    /nix/store/l38jrbilw269drpjkx7kinhrxj6fjh59-pan-0.14.2.91/bin/pan

  At most one of them can be active any given time.  Assuming than
  0.14.2.91 is active, you can active 0.128 as follows:

    $ nix-env --set-flag active false pan-0.14.2.91
    $ nix-env --set-flag active true pan-0.128

    $ readlink $(which pan)
    /nix/store/nziqwnlzy7xl385kglxhg75pfl5i936n-pan-0.128/bin/pan

  More flags to follow.

1 files changed, 3 insertions, 0 deletions

diff --git a/corepkgs/buildenv/default.nix b/corepkgs/buildenv/default.nix
index 9bc704d8d511..a5452db5ea89 100644
--- a/corepkgs/buildenv/default.nix
+++ b/corepkgs/buildenv/default.nix
@@ -6,4 +6,7 @@ derivation {
   builder = ./builder.pl;
   derivations = derivations;
   manifest = manifest;
+
+  # !!! grmbl, need structured data for passing this in a clean way.
+  active = map (x: if x ? meta && x.meta ? active then x.meta.active else "true") derivations;
 }