return (Collector) EnumSetAccumulator.TO_IMMUTABLE_ENUM_SET;
  }

  private static <E extends Enum<E>>
      Collector<E, EnumSetAccumulator<E>, ImmutableSet<E>> toImmutableEnumSetGeneric() {
    return Collector.of(
        EnumSetAccumulator::new,
        EnumSetAccumulator::add,
        EnumSetAccumulator::combine,
        EnumSetAccumulator::toImmutableSet,