private function writeProtect()
    {
        // Exit unless sheet protection has been specified
        if (!$this->phpSheet->getProtection()->getSheet()) {
            return;
        }

        $record = 0x0012; // Record identifier
        $length = 0x0002; // Bytes to follow

        $fLock = 1; // Worksheet is protected

        $header = pack('vv', $record, $length);
        $data = pack('v', $fLock);

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