enemies.add("Joe");
    assertEquals(2, frenemies.size());
    assertEquals(1, immut.size());
    assertEquals(1, mut.size());
  }

  public void testDifference() {
    Set<String> friends = newHashSet("Tom", "Joe", "Dave");
    Set<String> enemies = newHashSet("Dick", "Harry", "Tom");

    Set<String> goodFriends = Sets.difference(friends, enemies);

    enemies.add("Joe");
    assertEquals(2, frenemies.size());
    assertEquals(1, immut.size());
    assertEquals(1, mut.size());
  }

  public void testDifference() {
    Set<String> friends = newHashSet("Tom", "Joe", "Dave");
    Set<String> enemies = newHashSet("Dick", "Harry", "Tom");

    Set<String> goodFriends = Sets.difference(friends, enemies);

            .add(Range.open(6, 7))
            .add(Range.singleton(8))
            .build();

    assertThat(RANGE_SET_ONE.intersection(RANGE_SET_TWO)).isEqualTo(expected);
  }

  public void testDifference() {
    RangeSet<Integer> expected =
        ImmutableRangeSet.<Integer>builder()
            .add(Range.openClosed(3, 4))
            .add(Range.open(8, 10))
            .add(Range.openClosed(15, 17))