public function getPublisherIdentifier() : string
    {
        if ($this->rangeInfo === null) {
            throw IsbnException::unknownGroup($this->isbn);
        }

        if ($this->rangeInfo->parts === null) {
            throw IsbnException::unknownRange($this->isbn);
        }

        return $this->rangeInfo->parts[$this->is13 ? 2 : 1];
    }