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.

  useful way to transfer the closure of a store path to another
  machine.

  These commands provide functionality previously possible through
  `nix-push --copy'.  However, they are much more convenient in many
  situations (though possibly less efficient).
  
  Example:
  $ nix-pack-closure /nix/store/hj232g1r...-subversion-1.3.0 > svn.closure
  (on another machine:)
  $ nix-unpack-closure < svn.closure

  Note that Subversion is added to the store, but not installed into a
  user environment.  One should do `nix-env -i
  /nix/store/hj232g1r...-subversion-1.3.0' for that.

  Another example: copy the application Azureus to the machine
  `scratchy' through ssh:
  
  $ nix-pack-closure $(which azureus) | ssh scratchy nix-unpack-closure

  build directory (TMPDIR, i.e., /tmp).  Fixes NIX-26.

  deletes a path even if it is reachable from a root.  However, it
  won't delete a path that still has referrers (since that would
  violate store invariants).

  Don't try this at home.  It's a useful hack for recovering from
  certain situations in a somewhat clean way (e.g., holes in closures
  due to disk corruption).

  specified paths from the Nix store.  However, this operation is
  safe: it refuses to delete anything that the garbage collector
  wouldn't delete.

  (even when it is interrupted by a signal).