    @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);
    }