parser (roughly 80x faster).

  The absolutely latest version of Bison (1.875c) is required for
  reentrant GLR support, as well as a recent version of Flex (say,
  2.5.31).  Note that most Unix distributions ship with the
  prehistoric Flex 2.5.4, which doesn't support reentrancy.

  any substitutes for the derivation.

  ATermMaps.  Found thanks to Valgrind!

* Updated the README.  Now it just refers to the manual.

  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 :-)