public function addExternalSheet(PHPExcel_Worksheet $pSheet, $iSheetIndex = null)
    {
        if ($this->sheetNameExists($pSheet->getTitle())) {
            throw new PHPExcel_Exception("Workbook already contains a worksheet named '{$pSheet->getTitle()}'. Rename the external sheet first.");
        }

        // count how many cellXfs there are in this workbook currently, we will need this below
        $countCellXfs = count($this->cellXfCollection);

        // copy all the shared cellXfs from the external workbook and append them to the current
        foreach ($pSheet->getParent()->getCellXfCollection() as $cellXf) {
            $this->addCellXf(clone $cellXf);
        }

        // move sheet to this workbook
        $pSheet->rebindParent($this);

        // update the cellXfs
        foreach ($pSheet->getCellCollection(false) as $cellID) {
            $cell = $pSheet->getCell($cellID);
            $cell->setXfIndex($cell->getXfIndex() + $countCellXfs);
        }

        return $this->addSheet($pSheet, $iSheetIndex);
    }