public function writeWorkbookRelationships(Spreadsheet $spreadsheet)
    {
        // Create XML writer
        $objWriter = null;
        if ($this->getParentWriter()->getUseDiskCaching()) {
            $objWriter = new XMLWriter(XMLWriter::STORAGE_DISK, $this->getParentWriter()->getDiskCachingDirectory());
        } else {
            $objWriter = new XMLWriter(XMLWriter::STORAGE_MEMORY);
        }

        // XML header
        $objWriter->startDocument('1.0', 'UTF-8', 'yes');

        // Relationships
        $objWriter->startElement('Relationships');
        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');

        // Relationship styles.xml
        $this->writeRelationship(
            $objWriter,
            1,
            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles',
            'styles.xml'
        );

        // Relationship theme/theme1.xml
        $this->writeRelationship(
            $objWriter,
            2,
            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/theme',
            'theme/theme1.xml'
        );

        // Relationship sharedStrings.xml
        $this->writeRelationship(
            $objWriter,
            3,
            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings',
            'sharedStrings.xml'
        );

        // Relationships with sheets
        $sheetCount = $spreadsheet->getSheetCount();
        for ($i = 0; $i < $sheetCount; ++$i) {
            $this->writeRelationship(
                $objWriter,
                ($i + 1 + 3),
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet',
                'worksheets/sheet' . ($i + 1) . '.xml'
            );
        }
        // Relationships for vbaProject if needed
        // id : just after the last sheet
        if ($spreadsheet->hasMacros()) {
            $this->writeRelationShip(
                $objWriter,
                ($i + 1 + 3),
                'http://schemas.microsoft.com/office/2006/relationships/vbaProject',
                'vbaProject.bin'
            );
            ++$i; //increment i if needed for an another relation
        }

        $objWriter->endElement();

        return $objWriter->getData();
    }