code: def _all_classes(self, classes):
          
          all_classes = {}
  
          def recurse(cls):
              all_classes[cls] = None
              for c in cls.__bases__:
                  if c not in all_classes:
                      recurse(c)
  
          for cls in classes:
              recurse(cls)
  
          return all_classes.keys()
docs: Return a list of all classes that are ancestors of *classes*.