anchor.diff:  { stdenv, fetchurl, fetchgit, freetype, pkgconfig, yasm, freefont_ttf, ffmpeg, libass
   , python3, docutils, which
  -, x11Support ? true, libX11 ? null, libXext ? null, mesa ? null
  +, x11Support ? true, libX11 ? null, libXext ? null, mesa ? null, libXxf86vm ? null
   , xineramaSupport ? true, libXinerama ? null
   , xvSupport ? true, libXv ? null
   , alsaSupport ? true, alsaLib ? null
  
positive.code: , useUnfreeCodecs ? false
  }:
  
  assert x11Support -> (libX11 != null && libXext != null && mesa != null);
  assert xineramaSupport -> (libXinerama != null && x11Support);
  assert xvSupport -> (libXv != null && x11Support);
  assert alsaSupport -> alsaLib != null;
  
negative.code: 
    buildInputs = with stdenv.lib;
      [ freetype pkgconfig ffmpeg libass docutils which ]
      ++ optionals x11Support [ libX11 libXext mesa libXxf86vm ]
      ++ optional alsaSupport alsaLib
      ++ optional xvSupport libXv
      ++ optional theoraSupport libtheora
  