private function writeSupbookInternal()
    {
        $record = 0x01AE; // Record identifier
        $length = 0x0004; // Bytes to follow

        $header = pack('vv', $record, $length);
        $data = pack('vv', $this->spreadsheet->getSheetCount(), 0x0401);

        return $this->writeData($header . $data);
    }