private function writeCFHeader()
    {
        $record = 0x01B0; // Record identifier
        $length = 0x0016; // Bytes to follow

        $numColumnMin = null;
        $numColumnMax = null;
        $numRowMin = null;
        $numRowMax = null;
        $arrConditional = [];
        foreach ($this->phpSheet->getConditionalStylesCollection() as $cellCoordinate => $conditionalStyles) {
            foreach ($conditionalStyles as $conditional) {
                if ($conditional->getConditionType() == Conditional::CONDITION_EXPRESSION
                        || $conditional->getConditionType() == Conditional::CONDITION_CELLIS) {
                    if (!in_array($conditional->getHashCode(), $arrConditional)) {
                        $arrConditional[] = $conditional->getHashCode();
                    }
                    // Cells
                    $arrCoord = Coordinate::coordinateFromString($cellCoordinate);
                    if (!is_numeric($arrCoord[0])) {
                        $arrCoord[0] = Coordinate::columnIndexFromString($arrCoord[0]);
                    }
                    if ($numColumnMin === null || ($numColumnMin > $arrCoord[0])) {
                        $numColumnMin = $arrCoord[0];
                    }
                    if ($numColumnMax === null || ($numColumnMax < $arrCoord[0])) {
                        $numColumnMax = $arrCoord[0];
                    }
                    if ($numRowMin === null || ($numRowMin > $arrCoord[1])) {
                        $numRowMin = $arrCoord[1];
                    }
                    if ($numRowMax === null || ($numRowMax < $arrCoord[1])) {
                        $numRowMax = $arrCoord[1];
                    }
                }
            }
        }
        $needRedraw = 1;
        $cellRange = pack('vvvv', $numRowMin - 1, $numRowMax - 1, $numColumnMin - 1, $numColumnMax - 1);

        $header = pack('vv', $record, $length);
        $data = pack('vv', count($arrConditional), $needRedraw);
        $data .= $cellRange;
        $data .= pack('v', 0x0001);
        $data .= $cellRange;
        $this->append($header . $data);
    }