public function range()
    {
        $args = func_get_args();
        $arg_num = count($args);

        if ($arg_num%2 != 0) {
            throw new \InvalidArgumentException('Number of args must be even', 1);
        }

        $value = '[';

        for ($i = 0; $i < $arg_num;) {
            $value .= self::sanitize($args[$i++]) . '-' . self::sanitize($args[$i++]);
        }

        $value .= ']';

        return $this->add($value);
    }