private function writeGuts()
    {
        $record = 0x0080; // Record identifier
        $length = 0x0008; // Bytes to follow

        $dxRwGut = 0x0000; // Size of row gutter
        $dxColGut = 0x0000; // Size of col gutter

        // determine maximum row outline level
        $maxRowOutlineLevel = 0;
        foreach ($this->phpSheet->getRowDimensions() as $rowDimension) {
            $maxRowOutlineLevel = max($maxRowOutlineLevel, $rowDimension->getOutlineLevel());
        }

        $col_level = 0;

        // Calculate the maximum column outline level. The equivalent calculation
        // for the row outline level is carried out in writeRow().
        $colcount = count($this->columnInfo);
        for ($i = 0; $i < $colcount; ++$i) {
            $col_level = max($this->columnInfo[$i][5], $col_level);
        }

        // Set the limits for the outline levels (0 <= x <= 7).
        $col_level = max(0, min($col_level, 7));

        // The displayed level is one greater than the max outline levels
        if ($maxRowOutlineLevel) {
            ++$maxRowOutlineLevel;
        }
        if ($col_level) {
            ++$col_level;
        }

        $header = pack('vv', $record, $length);
        $data = pack('vvvv', $dxRwGut, $dxColGut, $maxRowOutlineLevel, $col_level);

        $this->append($header . $data);
    }