Nix.  This is to prevent Berkeley DB from becoming wedged.

  Unfortunately it is not possible to throw C++ exceptions from a
  signal handler.  In fact, you can't do much of anything except
  change variables of type `volatile sig_atomic_t'.  So we set an
  interrupt flag in the signal handler and check it at various
  strategic locations in the code (by calling checkInterrupt()).
  Since this is unlikely to cover all cases (e.g., (semi-)infinite
  loops), sometimes SIGTERM may now be required to kill Nix.

  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.

* Use curl instead of wget.

  files.  Target location is no longer hard-coded; it accepts a number
  of URLs on the command line.

* `nix-install-package': compatibility fixes.

  verbosity level.

  recovery fails.

  (following the Usenix paper).

* Some CSS tweaks.

  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].