anchor.diff:  { stdenv
   , buildPythonPackage
   , fetchPyPi
  +, lib
   
   , cffi
   , libsodium
  
positive.code:   src = fetchPyPi {
      package = "PyNaCl";
      inherit version;
      sha256 = "a251393a93c4bd85e7a2c1c0e938dc8862743ae27d8def87c32b2888ccca049e";
    };
  
    propagatedBuildInputs = [
  
negative.code:     libsodium
    ];
  
    # Make sure we use the system libsodium
    postPatch = ''
      sed -i '/def use_system():/a\    return True' setup.py
    '';
  