public function id($prefix = '')
    {
        static $id = 0;
        ++$id;
        $result = '';
        $lookup = array('M' => 1000, 'CM' => 900, 'D' => 500, 'CD' => 400, 'C' => 100, 'XC' => 90, 'L' => 50, 'XL' => 40, 'X' => 10, 'IX' => 9, 'V' => 5, 'IV' => 4, 'I' => 1);
        $number = $id;
        if ($number < 100) {
            $lookup = array_slice($lookup, 4);
        }
        foreach ($lookup as $roman => $value) {
            $matches = intval($number / $value);
            $result .= str_repeat($roman, $matches);
            $number = $number % $value;
        }

        return $prefix.$result;
    }