anchor.diff:      ++ stdenv.lib.optional vpxSupport libvpx
       ++ stdenv.lib.optional x264Support x264
       ++ stdenv.lib.optional xvidSupport xvidcore
  +    ++ stdenv.lib.optional opusSupport libopus
       ++ stdenv.lib.optional vdpauSupport libvdpau
       ++ stdenv.lib.optional vaapiSupport libva
       ++ stdenv.lib.optional faacSupport faac
  
positive.code: assert vpxSupport -> libvpx != null;
  assert x264Support -> x264 != null;
  assert xvidSupport -> xvidcore != null;
  assert vdpauSupport -> libvdpau != null;
  assert vaapiSupport -> libva != null;
  assert faacSupport -> faac != null;
  
negative.code: , vpxSupport ? false, libvpx ? null
  , x264Support ? true, x264 ? null
  , xvidSupport ? true, xvidcore ? null
  , opusSupport ? true, libopus ? null
  , vdpauSupport ? true, libvdpau ? null
  , vaapiSupport ? true, libva ? null
  , faacSupport ? false, faac ? null
  