docs: def _toplevel(cls):
          """Find the top level of the chain we're in.
  
              For example, if we have:
              C inheriting from B inheriting from A inheriting from ClosureModel
              C._toplevel() will return A.
          """
          superclasses = (
              list(set(ClosureModel.__subclasses__()) &
                   set(cls._meta.get_parent_list()))
          )
          return next(iter(superclasses)) if superclasses else cls