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: 
    gevent = buildPythonPackage rec {
      name = "gevent-1.0.1";
      disabled = isPy3k || isPyPy;  # see https://github.com/surfly/gevent/issues/248
  
      src = fetchurl {
        url = "https://pypi.python.org/packages/source/g/gevent/${name}.tar.gz";
  