docs: def descendents(self, cls):
          """
          Returns a descendent list of documentation objects for `cls`,
          which must be a documentation object.
  
          The list will contain objects belonging to `pydoc.Class` or
          `pydoc.External`. Objects belonging to the former are exported
          classes either in this module or in one of its sub-modules.
          """
          if cls.cls == type or not hasattr(cls.cls, '__subclasses__'):
              # Is this right?
              return []
  
          downs = cls.cls.__subclasses__()
          return list(map(lambda c: self.find_class(c), downs))