* New query option: `--compare-versions' or `-c' to compare installed

  versions to available versions, or vice versa.

  For example, the following compares installed versions to available
  versions:

    $ nix-env -qc
    autoconf-2.59            = 2.59
    automake-1.9.4           < 1.9.6
    f-spot-0.0.10            - ?
    firefox-1.0.4            < 1.0.7
    ...

  I.e., there are newer versions available (in the current default Nix
  expression) for Automake and Firefox, but not for Autoconf, and
  F-Spot is missing altogether.

  Conversely, the available versions can be compared to the installed
  versions:

    $ nix-env -qac
    autoconf-2.59                  = 2.59
    automake-1.9.6                 > 1.9.4
    bash-3.0                       - ?
    firefox-1.0.7                  > 1.0.4
    ...

  Note that bash is available but no version of it is installed.

  If multiple versions are available for comparison, then the highest
  is used.  E.g., if Subversion 1.2.0 is installed, and Subversion
  1.1.4 and 1.2.3 are available, then `nix-env -qc' will print `<
  1.2.3', not `> 1.1.4'.

  If higher versions are available, the version column is printed in
  red (using ANSI escape codes).