anchor.diff:  { lib
   , buildPythonPackage
   , fetchPypi
  +, pythonOlder
   , substituteAll
  -, fetchpatch
  -, nose
   , six
   , withGraphviz ? true
   , graphviz
   , fontconfig
  +# Tests
  +, pytestCheckHook
  +, nose
   }:
   
   buildPythonPackage rec {
  
positive.code:   # circular dependency anytree → graphviz → pango → glib → gtk-doc → anytree
    doCheck = withGraphviz;
  
    checkPhase = ''
      runHook preCheck
  
      nosetests
  
      runHook postCheck
    '';
  
    meta = with lib; {
      description = "Powerful and Lightweight Python Tree Data Structure";
  
negative.code:     })
    ];
  
    propagatedBuildInputs = [
      six
    ];
  