private function _writeMsoDrawingGroup()
	{
		// write the Escher stream if necessary
		if (isset($this->_escher)) {
			$writer = new PHPExcel_Writer_Excel5_Escher($this->_escher);
			$data = $writer->close();

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

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

		} else {
			return '';
		}
	}