anchor.diff:  
     propagatedBuildInputs = [ notebook nbdime git ];
   
  +  # all Tests on darwin fail or are skipped due to sandbox
  +  doCheck = !stdenv.isDarwin;
  +
     checkInputs = [ pytest ];
   
     checkPhase = ''
  
positive.code: { lib
  , buildPythonPackage
  , fetchPypi
  , pythonOlder
  
negative.code: 
    pythonImportsCheck = [ "jupyterlab_git" ];
  
    meta = with stdenv.lib; {
      description = "Jupyter lab extension for version control with Git.";
      license = with licenses; [ bsd3 ];
      homepage = "https://github.com/jupyterlab/jupyterlab-git";
  