anchor.diff:  { stdenv, lib, buildPythonPackage, fetchPypi, isPy27
   , nose, chai, simplejson, backports_functools_lru_cache
  -, python-dateutil, pytz, pytest-mock, sphinx, dateparser, pytestcov, pytest
  +, python-dateutil, pytz, pytest-mock, sphinx, dateparser, pytestcov
  +, pytestCheckHook
   }:
   
   buildPythonPackage rec {
  
positive.code:     sphinx
    ];
  
    checkPhase = ''
      pytest
    '';
  
    meta = with lib; {
      description = "Python library for date manipulation";
  
negative.code: 
    checkInputs = [
      dateparser
      pytestCheckHook
      pytestcov
      pytest-mock
      pytz
  