anchor.diff:  , enableLastfm         ? true
   , enableMpd            ? true
   , enableReplaygain     ? true, bs1770gain ? null
  +, enableSonosUpdate    ? true
   , enableThumbnails     ? true
   , enableWeb            ? true
   
  
positive.code:     mpdstats = enableMpd;
      mpdupdate = enableMpd;
      replaygain = enableReplaygain;
      thumbnails = enableThumbnails;
      web = enableWeb;
    };
  
negative.code: assert enableLastfm      -> pythonPackages.pylast         != null;
  assert enableMpd         -> pythonPackages.mpd2           != null;
  assert enableReplaygain  -> bs1770gain                    != null;
  assert enableSonosUpdate -> pythonPackages.soco           != null;
  assert enableThumbnails  -> pythonPackages.pyxdg          != null;
  assert enableWeb         -> pythonPackages.flask          != null;
  
  