anchor.diff:  , x11grabSupport ? false, libXext ? null, libXfixes ? null
   }:
   
  +assert jackSupport -> jackaudio != null;
   assert speexSupport -> speex != null;
   assert theoraSupport -> libtheora != null;
   assert vorbisSupport -> libvorbis != null;
  
positive.code: { stdenv, fetchurl, pkgconfig, yasm, zlib, bzip2, alsaLib
  , mp3Support ? true, lame ? null
  , speexSupport ? true, speex ? null
  , theoraSupport ? true, libtheora ? null
  
negative.code:     ++ stdenv.lib.optional x11grabSupport "--enable-x11grab";
  
    buildInputs = [ pkgconfig lame yasm zlib bzip2 alsaLib ]
      ++ stdenv.lib.optional jackSupport jackaudio
      ++ stdenv.lib.optional mp3Support lame
      ++ stdenv.lib.optional speexSupport speex
      ++ stdenv.lib.optional theoraSupport libtheora
  