Add the appropriate methods to *element_cls*.

def populate_class_members(self, element_cls, prop_name):
        """
        Add the appropriate methods to *element_cls*.
        """
        super(ZeroOrOneChoice, self).populate_class_members(
            element_cls, prop_name
        )
        self._add_choice_getter()
        for choice in self._choices:
            choice.populate_class_members(
                element_cls, self._prop_name, self._successors
            )
        self._add_group_remover()