anchor.diff:    name = "opengl-headers-${version}";
   
     src = fetchurl {
  -    url = "http://opengl-headers.tar.xz";
  -    multihash = "QmZvWAAPb4FAB4psCuvfKZW1N638AFG7ZTLX1Jr4kRE8UB";
  -    sha256 = "77172528da0549cebdaf4f5f129e020bfad722fdf9808f5e32d14766c2e81081";
  +    url = "http://opengl-headers.tar.xz";  # dummy url
  +    multihash = "QmY1EgsrwD8RjC1S6AWavDUjKWZ1pAafovQb97FaFedM1G";
  +    sha256 = "f79976ac9731f02b515c2eef164443acdaf34cc6ad9e60a6165e50ac82bac5fe";
     };
   
     configurePhase = ":";
  
positive.code:         sha256 = "40b8204a8c97e95913c31d11dc58527780e866424c5dd577f9a8ee2209612209";
        };
  
        configurePhase = ":";
  
        buildPhase = ''
          tar -Jcvf opengl-headers-${version}.tar.xz api/
        '';
  
  
negative.code: , fetchurl
  , fetchFromGitHub
  , lib
  , python2Packages
  }:
  
  # TODO: build release tarballs, repo vendors pdfs
  