protected function num_parts_correct(question_attempt $qa) {
        $a = new stdClass();
        list($a->num, $a->outof) = $qa->get_question()->get_num_parts_right(
                $qa->get_last_qt_data());
        if (is_null($a->outof)) {
            return '';
        } else {
            return get_string('yougotnright', 'question', $a);
        }
    }