'Add the appropriate methods to *element_cls*.'

def populate_class_members(self, element_cls, prop_name):

    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()
