anchor.diff:  
     buildInputs = [
       ffmpeg freetype libass libpthreadstubs
  -    lua lua5_sockets libuchardet rubberband
  +    lua lua5_sockets libuchardet
     ] ++ optional alsaSupport        alsaLib
       ++ optional xvSupport          libXv
       ++ optional theoraSupport      libtheora
  
positive.code:     ++ optional bluraySupport      libbluray
      ++ optional jackaudioSupport   libjack2
      ++ optional pulseSupport       libpulseaudio
      ++ optional screenSaverSupport libXScrnSaver
      ++ optional vdpauSupport       libvdpau
      ++ optional speexSupport       speex
  
negative.code: 
  with stdenv.lib;
  
  let
    available = x: x != null;
  in
  assert x11Support         -> all available [mesa libX11 libXext libXxf86vm];
  assert waylandSupport     -> all available [wayland libxkbcommon];
  assert rubberbandSupport  -> available rubberband;
  assert xineramaSupport    -> x11Support && available libXinerama;
  assert xvSupport          -> x11Support && available libXv;
  assert sdl2Support        -> available SDL2;
  