docs: static public void xA(double[] result, double[] x, double[][] A) {
      for (int i = 0; i < result.length; i++) {
        result[i] = 0.0;
        for (int j = 0; j < result.length; j++) {
          result[i] += (x[j] * A[j][i]);
        }
      }
    }