public function garbageCollect()
    {
        // how many references are there to each cellXf ?
        $countReferencesCellXf = array();
        foreach ($this->_cellXfCollection as $index => $cellXf) {
            $countReferencesCellXf[$index] = 0;
        }

        foreach ($this->getWorksheetIterator() as $sheet) {

            // from cells
            foreach ($sheet->getCellCollection(false) as $cellID) {
                $cell = $sheet->getCell($cellID);
                ++$countReferencesCellXf[$cell->getXfIndex()];
            }

            // from row dimensions
            foreach ($sheet->getRowDimensions() as $rowDimension) {
                if ($rowDimension->getXfIndex() !== null) {
                    ++$countReferencesCellXf[$rowDimension->getXfIndex()];
                }
            }

            // from column dimensions
            foreach ($sheet->getColumnDimensions() as $columnDimension) {
                ++$countReferencesCellXf[$columnDimension->getXfIndex()];
            }
        }

        // remove cellXfs without references and create mapping so we can update xfIndex
        // for all cells and columns
        $countNeededCellXfs = 0;
        foreach ($this->_cellXfCollection as $index => $cellXf) {
            if ($countReferencesCellXf[$index] > 0 || $index == 0) { // we must never remove the first cellXf
                ++$countNeededCellXfs;
            } else {
                unset($this->_cellXfCollection[$index]);
            }
            $map[$index] = $countNeededCellXfs - 1;
        }
        $this->_cellXfCollection = array_values($this->_cellXfCollection);

        // update the index for all cellXfs
        foreach ($this->_cellXfCollection as $i => $cellXf) {
            $cellXf->setIndex($i);
        }

        // make sure there is always at least one cellXf (there should be)
        if (empty($this->_cellXfCollection)) {
            $this->_cellXfCollection[] = new PHPExcel_Style();
        }

        // update the xfIndex for all cells, row dimensions, column dimensions
        foreach ($this->getWorksheetIterator() as $sheet) {

            // for all cells
            foreach ($sheet->getCellCollection(false) as $cellID) {
                $cell = $sheet->getCell($cellID);
                $cell->setXfIndex( $map[$cell->getXfIndex()] );
            }

            // for all row dimensions
            foreach ($sheet->getRowDimensions() as $rowDimension) {
                if ($rowDimension->getXfIndex() !== null) {
                    $rowDimension->setXfIndex( $map[$rowDimension->getXfIndex()] );
                }
            }

            // for all column dimensions
            foreach ($sheet->getColumnDimensions() as $columnDimension) {
                $columnDimension->setXfIndex( $map[$columnDimension->getXfIndex()] );
            }

			// also do garbage collection for all the sheets
            $sheet->garbageCollect();
        }
    }