private static function merge_multiple($option, $lists)
    {
        if (empty($lists)) {
            return null;
        }
        // found format using sprintf.
        if (isset($option['format'])) {
            $param = array_merge(array($option['format']), $lists);
            $found = call_user_func_array('sprintf', $param);
        } else {
            $con   = Helper::arrGet($option, 'connector', '-');
            $found = implode($con, $lists);
        }

        return $found;
    }