public function freezePane($cell, $topLeftCell = null)
    {
        if (is_string($cell) && Coordinate::coordinateIsRange($cell)) {
            throw new Exception('Freeze pane can not be set on a range of cells.');
        }

        if ($cell !== null && $topLeftCell === null) {
            $coordinate = Coordinate::coordinateFromString($cell);
            $topLeftCell = $coordinate[0] . $coordinate[1];
        }

        $this->freezePane = $cell;
        $this->topLeftCell = $topLeftCell;

        return $this;
    }