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

  public void testImmutableCopy_empty() {
    assertThat(union(emptySet(), emptySet()).immutableCopy()).isEmpty();
    assertThat(intersection(newHashSet(1, 2), newHashSet(3, 4)).immutableCopy()).isEmpty();