set1.add(3);
    assertThat(intersection).containsExactly(2, 3);
    set2.add(1);
    assertThat(intersection).containsExactly(1, 2, 3);
  }

  public void testDifference_isView() {
    Set<Integer> set1 = newHashSet(1, 2);
    Set<Integer> set2 = newHashSet(2, 3);
    Set<Integer> difference = difference(set1, set2);
    assertThat(difference).containsExactly(1);
    set1.add(0);