private function writeMsoDrawingGroup()
    {
        // write the Escher stream if necessary
        if (isset($this->escher)) {
            $writer = new Escher($this->escher);
            $data = $writer->close();

            $record = 0x00EB;
            $length = strlen($data);
            $header = pack('vv', $record, $length);

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

        return '';
    }