* Added an installation source `--from-expression' (or `-E') to

  install derivations from a Nix expression specified on the command
  line.  This is particularly useful for disambiguation if there are
  multiple derivations with the same name.  For instance, in Nixpkgs,
  to install the Firefox wrapper rather than the plain Firefox
  component:

    $ nix-env -f .../i686-linux.nix -i -E 'x: x.firefoxWrapper'

  The Nix expressions should be functions to which the default Nix
  expression (in this case, `i686-linux.nix') is passed, hence `x:
  ...'.

  This might also be a nice way to deal with high-level (user-level)
  variability, e.g.,

    $ nix-env -f ./server.nix -i -E 'x: x {port = 8080; ssl = false;}'
    

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