code: 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;
      }
docs: Gets sub matrix from current.
  
  This is usefull for determinant calculus, inverse, etc.
  
  @param  integer $int_m Index of rows to ignore
  @param  integer $int_n Index of column to ignore
  @access public
  @return Matrix