anchor.diff:  
       buildInputs = [ pkgs.openssl self.pretend self.cryptography_vectors
                       self.iso8601 self.pyasn1 self.pytest ];
  -    propagatedBuildInputs = [ self.six self.cffi  ];
  +    propagatedBuildInputs = [ self.six ] ++ optional (!isPyPy) self.cffi; 
     };
   
     cryptography_vectors = buildPythonPackage rec {
  
positive.code:   };
  
    coverage = buildPythonPackage rec {
      name = "coverage-3.7";
  
      src = pkgs.fetchurl {
        url = "http://pypi.python.org/packages/source/c/coverage/${name}.tar.gz";
        md5 = "055d82e6849d882ec6cf2ae1faca8e56";
      };
  
      meta = {
  
negative.code: 
  
    scikitlearn = buildPythonPackage {
      name = "scikit-learn-0.16.1";
  
      src = pkgs.fetchurl {
        url = "https://pypi.python.org/packages/source/s/scikit-learn/scikit-learn-0.15.2.tar.gz";
        sha256 = "19jzmbi3j4ix8418i80ayl595dwyi4gy474kb2nc1v8kdwgqi2hs";
      };
  
      buildInputs = with self; [ nose pillow pkgs.gfortran pkgs.glibcLocales ];
  