anchor.diff:  
     pyuv = buildPythonPackage rec {
       name = "pyuv-0.11.5";
  +    disabled = isPyPy;  # see https://github.com/saghul/pyuv/issues/49
   
       src = fetchurl {
         url = "https://github.com/saghul/pyuv/archive/${name}.tar.gz";
  
positive.code:     };
  
      buildInputs = [ pkgs.libev ];
      propagatedBuildInputs = [ greenlet ];
  
      meta = with stdenv.lib; {
        description = "Coroutine-based networking library";
  
negative.code: 
      buildInputs = [ nose httplib2  ];
  
      propagatedBuildInputs = optionals (!isPyPy) [ greenlet ];
  
      PYTHON_EGG_CACHE = "`pwd`/.egg-cache";
  
  