@Test
public void testCatenate() {

    IntList A = IntList.of(1, 2, 3);
    IntList B = IntList.of(4, 5, 6);
    IntList exp = IntList.of(1, 2, 3, 4, 5, 6);
    assertEquals(exp, IntList.catenate(A, B));
    assertEquals(IntList.of(1, 2, 3), A);
}