anchor.diff:      ++ optional  enableJemalloc  jemalloc
       ++ optional  enableHwAccel   libGLU_combined
       ++ optionals enablePlugins   [ xulrunner npapi_sdk ]
  -    ++ optionals enableGTK       [ gtk2 gnome2.gtkglext gnome2.GConf ]
  -    ++ optionals enableGstreamer [ gst-plugins-base gst-plugins-ugly gst-ffmpeg ];
  +    ++ optionals enableGTK       [ gtk2 gnome2.gtkglext gnome2.GConf ];
   
     configureFlags = with stdenv.lib; [
       "--with-boost-incl=${boost.dev}/include"
  
positive.code: 
  # media libraries
  assert enableFFmpeg    -> available ffmpeg_2 ;
  assert enableGstreamer -> all available [ gst-plugins-base gst-plugins-ugly gst-ffmpeg ];
  
  # misc
  assert enableJemalloc -> available jemalloc;
  
negative.code:   available = x: x != null;
  
    sound =
      if enableFFmpeg then "ffmpeg" else "none";
  
    renderers = []
      ++ optional enableAGG    "agg"
  