protected function storeBof($type)
    {
        $record = 0x0809; // Record identifier    (BIFF5-BIFF8)
        $length = 0x0010;

        // by inspection of real files, MS Office Excel 2007 writes the following
        $unknown = pack('VV', 0x000100D1, 0x00000406);

        $build = 0x0DBB; //    Excel 97
        $year = 0x07CC; //    Excel 97

        $version = 0x0600; //    BIFF8

        $header = pack('vv', $record, $length);
        $data = pack('vvvv', $version, $type, $build, $year);
        $this->append($header . $data . $unknown);
    }