Add the appropriate methods to *element_cls*.

def populate_class_members(
            self, element_cls, group_prop_name, successors):
        """
        Add the appropriate methods to *element_cls*.
        """
        self._element_cls = element_cls
        self._group_prop_name = group_prop_name
        self._successors = successors

        self._add_getter()
        self._add_creator()
        self._add_inserter()
        self._add_adder()
        self._add_get_or_change_to_method()