protected function _hex()
    {
        if(!$this->_decimal()->zero) {
            throw new \RuntimeException(
                'Cannot get hexadecimal numbers from non integer.'
            );
        }

        if($this->value < 0){
            return new S('-' . dechex(abs($this->value)));
        } else {
            return new S(dechex($this->value));
        }
    }