public static function get_submissions_parameters() {
        return new external_function_parameters(
            array(
                'assignmentids' => new external_multiple_structure(
                    new external_value(PARAM_INT, 'assignment id'),
                    '1 or more assignment ids',
                    VALUE_REQUIRED),
                'status' => new external_value(PARAM_ALPHA, 'status', VALUE_DEFAULT, ''),
                'since' => new external_value(PARAM_INT, 'submitted since', VALUE_DEFAULT, 0),
                'before' => new external_value(PARAM_INT, 'submitted before', VALUE_DEFAULT, 0)
            )
        );
    }