private function _writeExternsheet($sheetname)
	{
		$record      = 0x0017;                     // Record identifier
		$length      = 0x02 + strlen($sheetname);  // Number of bytes to follow

		$cch         = strlen($sheetname);         // Length of sheet name
		$rgch        = 0x03;                       // Filename encoding

		$header      = pack("vv",  $record, $length);
		$data        = pack("CC", $cch, $rgch);
		$this->_append($header . $data . $sheetname);
	}