default -> default-94-link
  default-82-link -> /nix/store/cc4480...
  default-83-link -> /nix/store/caeec8...
  ...
  default-94-link -> /nix/store/2896ca...
  experimental -> experimental-2-link
  experimental-1-link -> /nix/store/cc4480...
  experimental-2-link -> /nix/store/a3148f...

* `--profile' / `-p' -> `--switch-profile' / `-S'
* `--link' / `-l' -> `--profile' / `-p'
* The default profile is stored in $prefix/var/nix/profiles.
  $prefix/var/nix/links is gone.  Profiles can be stored anywhere.
* The current profile is now referenced from ~/.nix-profile, not
  ~/.nix-userenv.
* The roots to the garbage collector now have extension `.gcroot', not
  `.id'.

* Various performance improvements in the evaluator.
* Do not link against unused (and missing!) libraries (-lsglr, etc.).

* `--version' flag for all commands.
* Manual updates.

  any substitutes for the derivation.

  the output path of a derivation, not the path of its store
  expression.  This ensures that changes that affect the path of the
  store expression but not the output path, do not affect the
  `installed' state of a derivation.

  path of the Nix expression to be used with the import, upgrade, and
  query commands.  For instance,

  $ nix-env -I ~/nixpkgs/pkgs/system/i686-linux.nix

  $ nix-env --query --available   [aka -qa]
  sylpheed-0.9.7
  bison-1.875
  pango-1.2.5
  subversion-0.35.1
  ...

  $ nix-env -i sylpheed

  $ nix-env -u subversion

  There can be only one default at a time.

* If the path to a Nix expression is a symlink, follow the symlink
  prior to resolving relative path references in the expression.

  the symlink ~/.nix-userenv to the given argument (which defaults to
  .../links/current).  /etc/profile.d/nix-profile creates this symlink
  if it doesn't exist yet.  Example use:

  $ nix-env -l my_profile -i foo.nix subversion quake
  $ nix-env -p my_profile

  I don't like the term "profile".  Let's deprecate it :-)

    nix-env -u foo.nix strategoxt

  to replace the installed `strategoxt' derivation with the one from `foo.nix', if 
  the latter has a higher version number.  This is a no-op if `strategoxt' is not 
  installed.  Wildcards are also accepted, so

    nix-env -u foo.nix '*'

  will replace any installed derivation with newer versions from `foo.nix', if 
  available.

  The notion of "version number" is somewhat ad hoc, but should be useful in most 
  cases, as evidenced by the following unit tests for the version comparator:

    TEST("1.0", "2.3", -1);
    TEST("2.1", "2.3", -1);
    TEST("2.3", "2.3", 0);
    TEST("2.5", "2.3", 1);
    TEST("3.1", "2.3", 1);
    TEST("2.3.1", "2.3", 1);
    TEST("2.3.1", "2.3a", 1);
    TEST("2.3pre1", "2.3", -1);
    TEST("2.3pre3", "2.3pre12", -1);
    TEST("2.3a", "2.3c", -1);
    TEST("2.3pre1", "2.3c", -1);
    TEST("2.3pre1", "2.3q", -1);

  (-1 = less, 0 = equal, 1 = greater)

* A new verbosity level `lvlInfo', between `lvlError' and `lvlTalkative'.  This is 
  the default for `nix-env', so without any `-v' flags users should get useful 
  output, e.g.,

$ nix-env -u foo.nix strategoxt
upgrading `strategoxt-0.9.2' to `strategoxt-0.9.3'

    nix-env -i foo.nix subversion

  The version number part of a derivation name is defined as everything following the 
  first dash not followed by a letter.

* `--link' / `-l' flag to specify the switch symlink to use (by default, 
  /nix/var/nix/links/current).

$ nix-instantiate --help
error: unknown flag `--help`
Try `nix-instantiate --help' for more information.

:-)

* Hack: `nix-env -i *' installs all available derivations.

  configuration.
* Status queries.

* Replace all directory reading code by a generic readDirectory()
  function.