anchor.diff:        name = "opengl-headers-${version}";
   
         src = fetchFromGitHub {
  -        version = 3;
  +        version = 6;
           owner = "KhronosGroup";
           repo = "OpenGL-Registry";
  -        rev = "93e0595941ea275b95ba115e1f400283c652004d";
  -        sha256 = "40b8204a8c97e95913c31d11dc58527780e866424c5dd577f9a8ee2209612209";
  +        rev = "68dba34a93b67d626b1c8b7294e4562bdaf4c996";
  +        sha256 = "6046f2eef181fe96379d23938de26e32621a59061d384b946db7e969bf16e99a";
         };
   
         nativeBuildInputs = [
  
positive.code:       buildPhase = ''
          # Some headers such as glx.h are not pre-generated, regenerate all
          # to be sure none are missing.
          pushd xml
            python genheaders.py
          popd
          pushd api
            for header in glcorearb.h glext.h gl.h; do
              python ../xml/genglvnd.py -registry ../xml/gl.xml GL/$header.h
            done
          popd
  
          tar -Jcvf opengl-headers-${version}.tar.xz api/
        '';
  
negative.code: # TODO: build release tarballs, repo vendors pdfs
  
  let
    version = "2019-03-01";
  in
  stdenv.mkDerivation rec {
    name = "opengl-headers-${version}";
  
    src = fetchurl {
      url = "http://opengl-headers.tar.xz";  # dummy url
      multihash = "QmZdRTzsSxGWTSzmChywUgo6drKFoho8qDh4FagumAM3eS";
      sha256 = "7cfa0ed4091b4d04c33b1be3c30361f9613b7301744f0b4b6397b923d82295d0";
    };
  
    configurePhase = ":";
  