public function range() {
        $arg_num = func_num_args();
        if ($arg_num % 2 != 0) {
            throw new \InvalidArgumentException("Number of args must be even", 1);
        }
        $value = "[";
        $arg_list = func_get_args();
        for ($i = 0; $i < $arg_num;) {
            $value .= self::sanitize($arg_list[$i++]) . "-" . self::sanitize($arg_list[$i++]);
        }
        $value .= "]";
        return $this->add($value);
    }