assertThat(difference(newHashSet(1, 2), newHashSet(1, 2)).immutableCopy()).isEmpty();
    assertThat(symmetricDifference(newHashSet(1, 2), newHashSet(1, 2)).immutableCopy()).isEmpty();
  }

  public void testImmutableCopy() {
    assertThat(union(newHashSet(1, 2), newHashSet(2, 3)).immutableCopy()).containsExactly(1, 2, 3);
    assertThat(intersection(newHashSet(1, 2), newHashSet(2, 3)).immutableCopy()).containsExactly(2);