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'.

  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 :-)

  dynamically links against libdb4 (?!), due to LD_LIBRARY_PATH it picks 
  up our libdb4 instead of SuSE's libdb4, but our libdb4 uses another 
  glibc so loading barfs.

  Instead, all packages should use rpaths to store library locations in
  executables/libraries.  The disadvantage is that overriding rpaths is 
  harder.  (It is possible by invoking the dynamic linker directly, e.g., 
  `/lib/ld-linux.so.2 --ignore-path LIST program args...' to ignore the 
  rpath for the libraries in LIST).  It would be better to use DT_RUNPATH, 
  which is consulted by the dynamic linker *after* LD_LIBRARY_PATH but 
  *before* ld.so.cache and the system directories.

  etc. correctly.
* Fixed nix-switch.

  won't see header files under Nix control.

  build action for `system' packages (like system.fix) that have
  dependencies on all packages we want to activate.

  So the command sequence to switch to a new activation configuration
  of the system would be:

    $ fix -i .../fixdescriptors/system.fix
    ...
    system.fix -> 89cf4713b37cc66989304abeb9ea189f

    $ nix-switch 89cf4713b37cc66989304abeb9ea189f

* A nix-profile.sh script that can be included in .bashrc.