EnumSetAccumulator::combine,
        EnumSetAccumulator::toImmutableSet,
        Collector.Characteristics.UNORDERED);
  }

  @IgnoreJRERequirement // see enclosing class (whose annotation Animal Sniffer ignores here...)
  private static final class EnumSetAccumulator<E extends Enum<E>> {
    @SuppressWarnings({"rawtypes", "unchecked"})
    static final Collector<Enum<?>, ?, ImmutableSet<? extends Enum<?>>> TO_IMMUTABLE_ENUM_SET =