docs: public void swapRows(int r1, int r2, Object tmp)
      {
          int col = columns();
          System.arraycopy(array, col * r1, tmp, 0, col);
          System.arraycopy(array, col * r2, array, col * r1, col);
          System.arraycopy(tmp, 0, array, col * r2, col);
      }