public function removeNamedRange($namedRange, PHPExcel_Worksheet $pSheet = null) {
        if ($pSheet === NULL) {
            if (isset($this->_namedRanges[$namedRange])) {
                unset($this->_namedRanges[$namedRange]);
            }
        } else {
            if (isset($this->_namedRanges[$pSheet->getTitle() . '!' . $namedRange])) {
                unset($this->_namedRanges[$pSheet->getTitle() . '!' . $namedRange]);
            }
        }
        return $this;
    }