anchor.diff:  , enableLastfm         ? true
   , enableMpd            ? true
   , enableReplaygain     ? true, bs1770gain ? null
  +, enableSonosUpdate    ? true
   , enableThumbnails     ? true
   , enableWeb            ? true
   
  
positive.code: assert enableLastfm      -> pythonPackages.pylast         != null;
  assert enableMpd         -> pythonPackages.mpd2           != null;
  assert enableReplaygain  -> bs1770gain                    != null;
  assert enableThumbnails  -> pythonPackages.pyxdg          != null;
  assert enableWeb         -> pythonPackages.flask          != null;
  
  
negative.code:     ++ optional enableKeyfinder     keyfinder-cli
      ++ optional enableLastfm        pythonPackages.pylast
      ++ optional enableMpd           pythonPackages.mpd2
      ++ optional enableSonosUpdate   pythonPackages.soco
      ++ optional enableThumbnails    pythonPackages.pyxdg
      ++ optional enableWeb           pythonPackages.flask
      ++ optional enableAlternatives  plugins.alternatives
  