anchor.diff:  # TODO: build release tarballs, repo vendors pdfs
   
   let
  -  version = "2017-09-10";
  +  version = "2019-03-01";
   in
   stdenv.mkDerivation rec {
     name = "opengl-headers-${version}";
   
     src = fetchurl {
       url = "http://opengl-headers.tar.xz";  # dummy url
  -    multihash = "QmY1EgsrwD8RjC1S6AWavDUjKWZ1pAafovQb97FaFedM1G";
  -    sha256 = "f79976ac9731f02b515c2eef164443acdaf34cc6ad9e60a6165e50ac82bac5fe";
  +    multihash = "QmZdRTzsSxGWTSzmChywUgo6drKFoho8qDh4FagumAM3eS";
  +    sha256 = "7cfa0ed4091b4d04c33b1be3c30361f9613b7301744f0b4b6397b923d82295d0";
     };
   
     configurePhase = ":";
  
positive.code: 
    installPhase = ''
      for api in GL{,ES{,2,3},SC{,2}}; do
        pushd $api
          while read header; do
            install -D -m644 -v $header $out/include/$(basename "$api")/$header
          done < <(find . -name "*.h" -printf '%P\n')
        popd
      done
    '';
  
  
negative.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/$header.h
            done
          popd
          for xml in xml/*.xml; do
            install -D -m644 -v "$xml" \
              api/xml/"$(basename "$xml")"
          done
  
          tar -Jcvf opengl-headers-${version}.tar.xz api/
        '';
  