public function addCol($arr_col)
    {
        if (isset($this->arr[0]) && (count($this->arr[0]) == $this->size->cols)) {
            throw new \OutOfRangeException(sprintf('You cannot add another column! Max number of columns is %d', $this->size->cols));
        }

        if (count($arr_col) != $this->size->rows) {
            throw new \InvalidArgumentException('New column must have same amout of rows than previous columns.');
        }

        $arr_col = array_values($arr_col); //to be sure to have index 0, 1, 2…

        foreach ($arr_col as $k => $v) {
            $this->arr[$k][] = $arr_col[$k];
        }

        return $this;
    }