docs: def populate_class_members(self, element_cls, prop_name):
          """
          Add the appropriate methods to *element_cls*.
          """
          super(ZeroOrOne, self).populate_class_members(element_cls, prop_name)
          self._add_getter()
          self._add_creator()
          self._add_inserter()
          self._add_adder()
          self._add_get_or_adder()
          self._add_remover()