* 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.

  creates a new process group but also a new session.  New sessions
  have no controlling tty, so child processes like ssh cannot open
  /dev/tty (which is bad).

  intended).  This ensures that any ssh child processes to remote
  machines are also killed, and thus the Nix process on the remote
  machine also exits.  Without this, the remote Nix process will
  continue until it exists or until its stdout buffer gets full and it
  locks up.  (Partially fixes NIX-35.)

  "No space left on device" instead of "Success").  Reported by Karina
  Olmos.