public function unprotectCells($pRange)
    {
        // Uppercase coordinate
        $pRange = strtoupper($pRange);

        if (isset($this->protectedCells[$pRange])) {
            unset($this->protectedCells[$pRange]);
        } else {
            throw new Exception('Cell range ' . $pRange . ' not known as protected.');
        }

        return $this;
    }