code: public static <E extends Enum<E>> Collector<E, ?, ImmutableSet<E>> toImmutableEnumSet() {
      return (Collector) Accumulator.TO_IMMUTABLE_ENUM_SET;
    }
docs: Returns a {@code Collector} that accumulates the input elements into a new {@code ImmutableSet}
  with an implementation specialized for enums. Unlike {@link ImmutableSet#toImmutableSet}, the
  resulting set will iterate over elements in their enum definition order, not encounter order.
  
  @since 21.0