assertThat(union).containsExactly(1, 2, 3);
    set1.add(0);
    assertThat(union).containsExactly(0, 1, 2, 3);
    set2.remove(3);
    assertThat(union).containsExactly(0, 1, 2);
  }

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