target no longer applies to any available release.  This is a
  partial fix for NIX-34 (when producing linear patch sequences
  between releases, the number of patches grows without bound).

* Print errors to STDERR.

* Test "nix-store -q --tree" as well.

  with the same name *and* version number, and pick the first one
  (this means that the order in which channels appear in
  ~/.nix-channels matters).  E.g.:

    $ nix-env ii aterm
    warning: there are multiple derivations named `aterm-2.4.2'; using the first one
    installing `aterm-2.4.2'

  foo.  Now it will only install the one with the highest version
  number.

  ("installing `foo'" followed by "uninstalling `foo'").

  through the new `gc-reserved-space' option.

  the disk is full (because to delete something from the Nix store, we
  need a Berkeley DB transaction, which takes up disk space).  Under
  normal operation, we make sure that there exists a file
  /nix/var/nix/db/reserved of 1 MB.  When running the garbage
  collector, we delete that file before we open the Berkeley DB
  environment.

  implementations of MD5, SHA-1 and SHA-256.  The main benefit is that
  we get assembler-optimised implementations of MD5 and SHA-1 (though
  not SHA-256 (at least on x86), unfortunately).  OpenSSL's SHA-1
  implementation on Intel is twice as fast as ours.

  on the Bison parse stack.  Otherwise, a garbage collect during
  parsing could lead to a crash.

  a conservative garbage collector that scans the stack and parts of
  the heap for pointers to ATerms.  This scan can touch uninitialised
  memory, which is harmless.  Use:

  $ valgrind --suppressions=aterm-gc.supp ...
  

  Example:

    $ nix-build ./all-packages.nix -A xlibs.libX11

  So finally it's easy to perform a test build of a Nix expression!

  derivation(s) we're interested, e.g.,

    $ nix-instantiate ./all-packages.nix --attr xlibs.libX11

  List elements can also be selected:

    $ nix-instantiate ./build-for-release.nix --attr 0.subversion

  This allows a non-ambiguous specification of a derivation.  Of
  course, this should also be added to nix-env and nix-build.

  provided that all arguments have defaults.