public function addColumn($content, $columnIndex = null, $rowIndex = null)
    {
        $rowIndex = $rowIndex === null
            ? $this->rowIndex
            : $rowIndex;
        if ($columnIndex === null) {
            $columnIndex = isset($this->rows[ $rowIndex ])
                ? count($this->rows[ $rowIndex ])
                : 0;
        }
        $this->rows[ $rowIndex ][ $columnIndex ] = $content;

        return $this;
    }