public void testToImmutableEnumSet() {
    Set<SomeEnum> units = Stream.of(SomeEnum.D, SomeEnum.B).collect(toImmutableEnumSet());

    assertThat(units).containsExactly(SomeEnum.B, SomeEnum.D).inOrder();
  }

  public void testToImmutableEnumSetEmpty() {
    Set<SomeEnum> units = Stream.<SomeEnum>empty().collect(toImmutableEnumSet());
    assertThat(units).isEmpty();
  }

  public void testToImmutableEnumSetReused() {

        ImmutableSortedSet.Builder::combine,
        ImmutableSortedSet.Builder::build);
  }

  @SuppressWarnings({"rawtypes", "unchecked"})
  static <E extends Enum<E>> Collector<E, ?, ImmutableSet<E>> toImmutableEnumSet() {
    return (Collector) EnumSetAccumulator.TO_IMMUTABLE_ENUM_SET;
  }

  private static <E extends Enum<E>>
      Collector<E, EnumSetAccumulator<E>, ImmutableSet<E>> toImmutableEnumSetGeneric() {

  @SuppressWarnings("Java7ApiChecker")
  @IgnoreJRERequirement // Users will use this only if they're already using streams.
  public static <E extends Enum<E>> Collector<E, ?, ImmutableSet<E>> toImmutableEnumSet() {
    return CollectCollectors.toImmutableEnumSet();
  }

  /**
   * Returns a new, <i>mutable</i> {@code EnumSet} instance containing the given elements in their