public function subMatrix($int_m, $int_n)
    {
        $sm = new self($this->size->rows - 1, $this->size->cols - 1);

        foreach ($this->arr as $m => $row) {
            if ($m != $int_m) {
                $arr_row = array();

                foreach ($row as $n => $v) {
                    if ($n != $int_n) {
                        $arr_row[] = $v;
                    }
                }

                $sm->addRow($arr_row);
            }
        }

        return $sm;
    }