docs: public void swapRows(int r1, int r2)
      {
          int n = columns();
          ItemSupplier s = supplier;
          ItemConsumer c = consumer;
          for (int j = 0; j < n; j++)
          {
              double v = s.get(r1, j);
              c.set(r1, j, s.get(r2, j));
              c.set(r2, j, v);
          }
      }