code: public static <T> ImmutableSortedSet<ImmutableSortedSet<T>> immutablePowerSet(SortedSet<T> set)
      {
          return powerSet(set).collect(new Function<MutableSortedSet<T>, ImmutableSortedSet<T>>()
          {
              public ImmutableSortedSet<T> valueOf(MutableSortedSet<T> set)
              {
                  return set.toImmutable();
              }
          }, TreeSortedSet.<ImmutableSortedSet<T>>newSet(Comparators.<T>powerSet())).toImmutable();
      }
docs: Returns an Immutable version of powerset where the inner sets are also immutable.