@Test
public void testXmy() {

    Matrix x = new Matrix(10, 10);
    Matrix m = new Matrix(10, 10);
    Matrix y = new Matrix(10, 10);
    x.randomize();
    m.randomize();
    y.randomize();
    Matrix z = x.times(m).times(y);
    m.subMatrix().xmy(x.subMatrix(), y.subMatrix());
    
    Matrix d = m.minus(z);
    assertTrue(d.nrm2() < 1e-9);
}