anchor.diff:      optionals x11Support [ xlibsWrapper libXrandr ] ++
       optional alsaSupport alsaLib ++
       optional stdenv.isLinux libcap ++
  -    optional openglSupport mesa_noglu ++
  +    optional openglSupport mesa ++
       optional pulseaudioSupport libpulseaudio ++
       optional stdenv.isDarwin Cocoa;
   
  
positive.code: # PulseAudio.
  assert (stdenv.isLinux && !(stdenv ? cross)) -> alsaSupport || pulseaudioSupport;
  
  assert openglSupport -> (mesa_noglu != null && x11Support);
  assert x11Support -> (xlibsWrapper != null && libXrandr != null);
  assert alsaSupport -> alsaLib != null;
  assert pulseaudioSupport -> libpulseaudio != null;
  
negative.code: { stdenv, fetchurl, fetchpatch, pkgconfig, audiofile, libcap
  , openglSupport ? false, mesa ? null
  , alsaSupport ? true, alsaLib ? null
  , x11Support ? true, xlibsWrapper ? null, libXrandr ? null
  , pulseaudioSupport ? true, libpulseaudio ? null
  