assertThat(symmetricDifference(newHashSet(1, 2), newHashSet(2, 3)).immutableCopy())
        .containsExactly(1, 3);
  }

  public void testCopyInto_returnsSameInstance() {
    Set<Object> set = new HashSet<>();
    assertThat(union(emptySet(), emptySet()).copyInto(set)).isSameInstanceAs(set);
    assertThat(intersection(emptySet(), emptySet()).copyInto(set)).isSameInstanceAs(set);