private static function attempt_structure() {
        return new external_single_structure(
            array(
                'id' => new external_value(PARAM_INT, 'Attempt id.', VALUE_OPTIONAL),
                'quiz' => new external_value(PARAM_INT, 'Foreign key reference to the quiz that was attempted.',
                                                VALUE_OPTIONAL),
                'userid' => new external_value(PARAM_INT, 'Foreign key reference to the user whose attempt this is.',
                                                VALUE_OPTIONAL),
                'attempt' => new external_value(PARAM_INT, 'Sequentially numbers this students attempts at this quiz.',
                                                VALUE_OPTIONAL),
                'uniqueid' => new external_value(PARAM_INT, 'Foreign key reference to the question_usage that holds the
                                                    details of the the question_attempts that make up this quiz
                                                    attempt.', VALUE_OPTIONAL),
                'layout' => new external_value(PARAM_RAW, 'Attempt layout.', VALUE_OPTIONAL),
                'currentpage' => new external_value(PARAM_INT, 'Attempt current page.', VALUE_OPTIONAL),
                'preview' => new external_value(PARAM_INT, 'Whether is a preview attempt or not.', VALUE_OPTIONAL),
                'state' => new external_value(PARAM_ALPHA, 'The current state of the attempts. \'inprogress\',
                                                \'overdue\', \'finished\' or \'abandoned\'.', VALUE_OPTIONAL),
                'timestart' => new external_value(PARAM_INT, 'Time when the attempt was started.', VALUE_OPTIONAL),
                'timefinish' => new external_value(PARAM_INT, 'Time when the attempt was submitted.
                                                    0 if the attempt has not been submitted yet.', VALUE_OPTIONAL),
                'timemodified' => new external_value(PARAM_INT, 'Last modified time.', VALUE_OPTIONAL),
                'timemodifiedoffline' => new external_value(PARAM_INT, 'Last modified time via webservices.', VALUE_OPTIONAL),
                'timecheckstate' => new external_value(PARAM_INT, 'Next time quiz cron should check attempt for
                                                        state changes.  NULL means never check.', VALUE_OPTIONAL),
                'sumgrades' => new external_value(PARAM_FLOAT, 'Total marks for this attempt.', VALUE_OPTIONAL),
            )
        );
    }