public double sub(int i, int j, double b) {
        return A[index(i, j)] -= b;
    }
@Test
    public void testSub() {
        System.out.println("sub");
        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 = {
                { 0.0338183,  0.1424245, -0.0338183f},
                {-0.6349342, -1.7808990,  0.6349342f},
                {-1.2632161,  0.8989516,  1.2632161f}
        };
        Matrix a = Matrix.of(A);
        Matrix b = Matrix.of(B);
        a.sub(b);
        assertTrue(MathEx.equals(C, a.toArray(), 1E-7));
    }