public static function get_grades_table_parameters() {
        return new external_function_parameters (
            array(
                'courseid' => new external_value(PARAM_INT, 'Course Id', VALUE_REQUIRED),
                'userid'   => new external_value(PARAM_INT, 'Return grades only for this user (optional)', VALUE_DEFAULT, 0),
                'groupid'  => new external_value(PARAM_INT, 'Get users from this group only', VALUE_DEFAULT, 0)
            )
        );
    }