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