public double add(int i, int j, double b) {
        long k = index(i, j);
        double y = A.get(k) + b;
        A.put(k, y);
        return y;
    }
@Test
    public void testAdd() {
        System.out.println("add");
        double[][] A = {
                { 0.7220180,  0.07121225, 0.6881997f},
                {-0.2648886, -0.89044952, 0.3700456f},
                {-0.6391588,  0.44947578, 0.6240573f}
        };
        double[][] B = {
                {0.6881997, -0.07121225,  0.7220180f},
                {0.3700456,  0.89044952, -0.2648886f},
                {0.6240573, -0.44947578, -0.6391588f}
        };
        double[][] C = {
                { 1.4102177, 0,  1.4102177f},
                { 0.1051570, 0,  0.1051570f},
                {-0.0151015, 0, -0.0151015f}
        };
        BigMatrix a = BigMatrix.of(A);
        BigMatrix b = BigMatrix.of(B);
        a.add(1.0, b);
        assertTrue(MathEx.equals(C, a.toArray(), 1E-7));
    }