protected function check_qt_var_name_restrictions($expected) {
        global $CFG;

        if ($CFG->debugdeveloper && $expected !== self::USE_RAW_DATA) {
            foreach ($expected as $key => $value) {
                if (strpos($key, 'bf_') !== false) {
                    debugging('The bf_ prefix is reserved and cannot be used by question types', DEBUG_DEVELOPER);
                }
            }
        }
    }