anchor.diff:  
     svg2tikz = self.buildPythonPackage {
       name = "svg2tikz-1.0.0";
  +    disabled = ! isPy27;
   
       propagatedBuildInputs = with self; [lxml];
   
  
positive.code: 
    networkx = buildPythonPackage rec {
      name = "networkx-1.9.1";
  
      src = pkgs.fetchurl {
        url = "https://pypi.python.org/packages/source/n/networkx/networkx-1.9.1.tar.gz";
  
negative.code:     };
    };
  
  }; in pythonPackages
  