anchor.diff:  { stdenv
   , buildPythonPackage
   , fetchPypi
  +, fetchpatch
   , isPy3k
   , setuptoolsDarcs
   , darcsver
  
positive.code: 
    src = fetchPypi {
      inherit pname version;
      sha256 = "08ad57a1a39b7ed23c173692281da0b8d49d98ad3dcc09f8cca6d901e142699f";
    };
  
    # Prefer crypto++ library from the Nix store over the one that's included
    # in the pycryptopp distribution.
  
negative.code:   buildInputs = [ setuptoolsDarcs darcsver pkgs.cryptopp ];
  
    meta = with stdenv.lib; {
      homepage = "https://tahoe-lafs.org/trac/pycryptopp";
      description = "Python wrappers for the Crypto++ library";
      license = licenses.gpl2Plus;
      platforms = platforms.linux;
  