public float sub(int i, int j, float b) {
        return A[index(i, j)] -= b;
    }
@Test
    public void testSub() {
        System.out.println("sub");
        float[][] A = {
                { 0.7220180f,  0.07121225f, 0.6881997f},
                {-0.2648886f, -0.89044952f, 0.3700456f},
                {-0.6391588f,  0.44947578f, 0.6240573f}
        };
        float[][] B = {
                {0.6881997f, -0.07121225f,  0.7220180f},
                {0.3700456f,  0.89044952f, -0.2648886f},
                {0.6240573f, -0.44947578f, -0.6391588f}
        };
        float[][] C = {
                { 0.0338183f,  0.1424245f, -0.0338183f},
                {-0.6349342f, -1.7808990f,  0.6349342f},
                {-1.2632161f,  0.8989516f,  1.2632161f}
        };
        Matrix a = Matrix.of(A);
        Matrix b = Matrix.of(B);
        a.sub(b);
        assertTrue(MathEx.equals(C, a.toArray(), 1E-6f));
    }