android/guava/src/com/google/common/collect/Sets.java
@GwtCompatible(serializable = true)
  public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(Iterable<E> elements) {
    if (elements instanceof ImmutableEnumSet) {
      return (ImmutableEnumSet<E>) elements;
    } else if (elements instanceof Collection) {
      Collection<E> collection = (Collection<E>) elements;
      if (collection.isEmpty()) {
        return ImmutableSet.of();
      } else {
        return ImmutableEnumSet.asImmutable(EnumSet.copyOf(collection));
      }
    } else {
      Iterator<E> itr = elements.iterator();
      if (itr.hasNext()) {
        EnumSet<E> enumSet = EnumSet.of(itr.next());
        Iterators.addAll(enumSet, itr);
        return ImmutableEnumSet.asImmutable(enumSet);
      } else {
        return ImmutableSet.of();
      }
    }
  }
@GwtCompatible(serializable = true)
  public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(Iterable<E> elements) {
    if (elements instanceof ImmutableEnumSet) {
      return (ImmutableEnumSet<E>) elements;
    } else if (elements instanceof Collection) {
      Collection<E> collection = (Collection<E>) elements;
      if (collection.isEmpty()) {
        return ImmutableSet.of();
      } else {
        return ImmutableEnumSet.asImmutable(EnumSet.copyOf(collection));
      }
    } else {
      Iterator<E> itr = elements.iterator();
      if (itr.hasNext()) {
        EnumSet<E> enumSet = EnumSet.of(itr.next());
        Iterators.addAll(enumSet, itr);
        return ImmutableEnumSet.asImmutable(enumSet);
      } else {
        return ImmutableSet.of();
      }
    }
  }
http://code.google.com/p/google-web-toolkit/issues/detail?id=3028