docs: public static DoubleMatrix multiply(DoubleMatrix m1, DoubleMatrix m2)
      {
          if (m1.cols != m2.rows)
          {
              throw new IllegalArgumentException("Matrices not comfortable");
          }
          int m = m1.rows;
          int n = m1.cols;
          int p = m2.cols;
          ItemSupplier s1 = m1.supplier;
          ItemSupplier s2 = m2.supplier;
          DoubleMatrix mr = DoubleMatrix.getInstance(m, p);
          ItemConsumer c = mr.consumer;
          for (int i = 0; i < m; i++)
          {
              for (int j = 0; j < p; j++)
              {
                  double s = 0;
                  for (int r = 0; r < n; r++)
                  {
                      s += s1.get(i, r) * s2.get(r, j);
                  }
                  c.set(i, j, s);
              }
          }
          return mr;
      }