anchor.diff:  { lib
   , buildPythonPackage
   , fetchPypi
  +, pythonOlder
   , substituteAll
  -, fetchpatch
  -, nose
   , six
   , withGraphviz ? true
   , graphviz
   , fontconfig
  +# Tests
  +, pytestCheckHook
  +, nose
   }:
   
   buildPythonPackage rec {
  
positive.code:     })
    ];
  
    checkInputs = [
      nose
    ];
  
    propagatedBuildInputs = [
      six
    ];
  
negative.code:   # circular dependency anytree → graphviz → pango → glib → gtk-doc → anytree
    doCheck = withGraphviz;
  
    checkInputs = [ pytestCheckHook nose ];
  
    pytestFlagsArray = lib.optionals (pythonOlder "3.4") [
      # Use enums, which aren't available pre-python3.4
      "--ignore=tests/test_resolver.py"
      "--ignore=tests/test_search.py"
    ];
  
    meta = with lib; {
      description = "Powerful and Lightweight Python Tree Data Structure";
  