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