$ nix-env -f . -qa '*' --meta --xml --drv-path --show-trace
error: while querying the derivation named `clementine-1.2.1':
while evaluating `optional' at .../lib/lists.nix:113:20, called from .../pkgs/applications/audio/clementine/default.nix:50:22:
undefined variable `not' at .../pkgs/applications/audio/clementine/default.nix:50:32
Clementine has an optional dependency on libspotify, which is unfree.
Enabling libspotify unconditionally prevented Hydra from distributing
Clementine. Now, we optionally enable it based on
config.clementine.spotify.
