private function writeNumberFormat($format, $ifmt)
    {
        $record = 0x041E; // Record identifier

        $numberFormatString = StringHelper::UTF8toBIFF8UnicodeLong($format);
        $length = 2 + strlen($numberFormatString); // Number of bytes to follow

        $header = pack('vv', $record, $length);
        $data = pack('v', $ifmt) . $numberFormatString;
        $this->append($header . $data);
    }