* Documented release procedures.

  is specified.

  Nix expressions.

  To subscribe to a channel (needs to be done only once):

    nix-channel --add \
      http://catamaran.labs.cs.uu.nl/dist/nix/channels/nixpkgs-unstable

  This just adds the given URL to ~/.nix-channels (which can also be
  edited manually).

  To update from all channels:

    nix-channel --update

  This fetches the latest expressions and pulls cache manifests.  The
  default Nix expression (~/.nix-defexpr) is made to point to the
  conjunction of the expressions downloaded from all channels.

  So to update all installed derivations in the current user
  environment:

    nix-channel --update
    nix-env --upgrade '*'

  If you are really courageous, you can put this in a cronjob or
  something.

  You can subscribe to multiple channels.  It is not entirely clear
  what happens when there are name clashes between derivations from
  different channels.  From nix-env/main.cc it appears that the one
  with the lowest (highest?) hash will be used, which is pretty
  meaningless.

  unless `--disable-init-state' is passed to configure.

  an expression.

  environment variable.  This is useful for passing authentication
  information (it won't show up in `ps').  Hacky - nix-push should
  abstract over the use of Curl.

  efficient by creating only a single symlink to entire directory
  trees unless a collission occurs.

* Made the dependencies on bzip2 and the shell explicit.

  print a nice backtrace of the stack, rather than vomiting a gigantic
  (and useless) aterm on the screen.  Example:

    error: while evaluating file `.../pkgs/system/test.nix':
    while evaluating attribute `subversion' at `.../pkgs/system/all-packages-generic.nix', line 533:
    while evaluating function at `.../pkgs/applications/version-management/subversion/default.nix', line 1:
    assertion failed at `.../pkgs/applications/version-management/subversion/default.nix', line 13

  Since the Nix expression language is lazy, the trace may be
  misleading.  The purpose is to provide a hint as to the location of
  the problem.    
  

  binding.

  instead of `derivation' triggered a huge slowdown in the Nix
  expression evaluator.  Total execution time of `nix-env -qa' went up
  by a factor of 60 or so.

  This scalability problem was caused by expressions such as

    (x: y: ... x ...) a b

  where `a' is a large term (say, the one in
  `all-packages-generic.nix').  Then the first beta-reduction would
  produce

    (y: ... a ...) b

  by substituting `a' for `x'.  The second beta-reduction would then
  substitute `b' for `y' into the body `... a ...', which is a large
  term due to `a', and thus causes a large traversal to be performed
  by substitute() in the second reduction.  This is however entirely
  redundant, since `a' cannot contain free variables (since we never
  substitute below a weak head normal form).

  The solution is to wrap substituted terms into a `Closed'
  constructor, i.e.,

    subst(subs, Var(x)) = Closed(e) iff subs[x] = e

  have substitution not descent into closed terms,

    subst(subs, Closed(x)) = Closed(x)

  and otherwise ignore them for evaluation,

    eval(Closed(x)) = eval(x).

* Fix a typo that caused incorrect substitutions to be performed in
  simple lambdas, e.g., `(x: x: x) a' would reduce to `(x: a)'.
    

  `attrs ? x' yields true iff `attrs' has an attribute named `x'.

    {stdenv, bash}: derivation {
      builder = bash ~ /bin/sh;
      args = ["-e" "-x" ./builder.sh];
      ...
    }

  Here the attribute `builder' will evaluate to, e.g.,
  `/nix/store/1234abcd...-bash-2.0.1/bin/sh'.

  `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.