it automatically removes log files when they are no longer needed.

  *** IMPORTANT ***

  If you have an existing Nix installation, you must checkpoint the
  Nix database to prevent recent transactions from being undone.  Do
  the following:

  - optional: make a backup of $prefix/var/nix/db.

  - run `db_checkpoint' from Berkeley DB 4.1:
  
    $ db_checkpoint -h $prefix/var/nix/db -1

  - optional (?): run `db_recover' from Berkeley DB 4.1:

    $ db_recover -h $prefix/var/nix/db

  - remove $prefix/var/nix/db/log* and $prefix/var/nix/db/__db*
    

  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 :-)

* Allow extra flags to be passed to RPM.

* Respect DESTDIR variable.

    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).

* Integrity: check in successor / substitute registration whether
  the target path exists or has a substitute.

  --register-[substitutes|successors].

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

:-)

  "i686-linux" instead of "i686-suse-linux").

* Convert tabs to spaces.