`bla:' is now no longer parsed as a URL.

* Re-enabled support for the `args' attribute in derivations to
  specify command line arguments to the builder, e.g.,

    ...
    builder = /usr/bin/python;
    args = ["-c" ./builder.py];
    ...

  This is because the contents of these symlinks are not incorporated
  into the hashes of derivations, and could therefore cause a mismatch
  between the build system and the target system.  E.g., if
  `/nix/store' is a symlink to `/data/nix/store', then a builder could
  expand this path and store the result.  If on the target system
  `/nix/store' is not a symlink, or is a symlink that points somewhere
  else, we have a dangling pointer.

  The trigger for this change is that gcc 3.3.3 does exactly that (it
  applies realpath() to some files, such as libraries, which causes
  our impurity checker to bail out.)

  An annoying side-effect of this change is that it makes it harder to
  move the Nix store to a different file system.  On Linux, bind
  mounts can be used instead of symlink for this purpose (e.g., `mount
  -o bind /data/nix/store /nix/store').

  called explicitly on a Nest object.

  writes to stderr:
  
  - `pretty': the old nested style (default)
  - `escapes': uses escape codes to indicate nesting and message
    level; can be processed using `log2xml'
  - `flat': just plain text, no nesting

  These can be set using `--log-type TYPE' or the NIX_LOG_TYPE
  environment variable.  

  unimportant messages, it is collapsed by the default.
* Also added an optional integer argument to the escape code for opening a nesting 
  level to indicate lack of importance.  If set, the tree is collapsed by default.

  representing tree structures.

  Martin and CSS guru Martijn Vermaat.

  Hovering over the abbreviated path will reveal the full path.  This
  probably only works in Mozilla. 

  build logs.  The program `log2xml' converts a Nix build log (read
  from standard input) into XML file that can then be converted to
  XHTML by the `log2html.xsl' stylesheet.  The CSS stylesheet
  `logfile.css' is necessary to make it look good.

  This is primarily useful if the log file has a *tree structure*,
  i.e., that sub-tasks such as the various phases of a build (unpack,
  configure, make, etc.) or recursive invocations of Make are
  represented as such.  While a log file is in principle an
  unstructured plain text file, builders can communicate this tree
  structure to `log2xml' by using escape sequences:

  - "\e[p" starts a new nesting level; the first line following the
    escape code is the header;

  - "\e[q" ends the current nesting level.

  The generic builder in nixpkgs (not yet committed) uses this.  It
  shouldn't be to hard to patch GNU Make to speak this protocol.

  Further improvements to the generated HTML pages are to allow
  collapsing/expanding of subtrees, and to abbreviate store paths (but
  to show the full path by hovering the mouse over it).
  

  builders to point to the store and the temporary build directory,
  respectively.  Useful for purity checking.
* Also set TEMPDIR, TMPDIR, TEMP, and TEMP to NIX_BUILD_TOP to make
  sure that tools in the builder store temporary files in the right
  location.

  b then x else y.z'.

* Do not create stuff in localstatedir when doing `make install'
  (since we may not have write access).  In general, installation of
  constant code/data should be separate from the initialisation of
  mutable state.

  non-inherited attributes.

  chroot() environment.
* A operation `--validpath' to register path validity.  Useful for
  bootstrapping in a pure Nix environment.
* Safety checks: ensure that files involved in store operations are in
  the store.

  Otherwise the substitute mechanism can break in subtle ways.

  derivation (i.e., the closure store expression) a root of the
  garbage collector.  This ensures that running `nix-collect-garbage
  --no-successors' is safe.

* nix-pull now requires the full url to the manifest, i.e.,
  `/MANIFEST/' is no longer automatically appended.
* nix-prefetch-url works again.  

  whether we want to upgrade if the current version is less than the
  available version (default), when it is less or equal, or always.
  
* Added a flag `--dry-run' to show what would happen in `--install',
  `--uninstall', and `--upgrade', without actually performing the
  operation.

  of the current profile, e.g.,

    $ nix-env --list-generations
    ...
    39   2004-02-02 17:53:53
    40   2004-02-02 17:55:18
    41   2004-02-02 17:55:41
    42   2004-02-02 17:55:50   (current)

    $ nix-env --switch-generation 39

    $ ls -l /nix/var/nix/profiles/default
    ... default -> default-39-link

* Also a command `--rollback' which is just a convenience operation to
  rollback to the oldest generation younger than the current one.

  Note that generations properly form a tree.  E.g., if after
  switching to generation 39, we perform an installation action,
  a generation 43 is created which is a descendant of 39, not 42.  So
  a rollback from 43 ought to go back to 39.  This is not currently
  implemented; generations form a linear sequence.

  profile.

  argument is specified.

  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'.

  other attribute sets, rather than the current scope.  E.g.,
  
    {inherit (pkgs) gcc binutils;}

  is equivalent to

    {gcc = pkgs.gcc; binutils = pkgs.binutils;}

  I am not so happy about the syntax.

  {x=1; y=2; z=3;} // {y=4;}  =>  {x=1; y=4; z=3;}