function _storeExternsheet($sheetname)
    {
        $record    = 0x0017;         // Record identifier

        // References to the current sheet are encoded differently to references to
        // external sheets.
        //
        if ($this->name == $sheetname) {
            $sheetname = '';
            $length    = 0x02;  // The following 2 bytes
            $cch       = 1;     // The following byte
            $rgch      = 0x02;  // Self reference
        } else {
            $length    = 0x02 + strlen($sheetname);
            $cch       = strlen($sheetname);
            $rgch      = 0x03;  // Reference to a sheet in the current workbook
        }

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