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() {