code: public static <E, S extends MutableSortedBag<E>> UnmodifiableSortedBag<E> of(S bag)
      {
          if (bag == null)
          {
              throw new IllegalArgumentException("cannot create an UnmodifiableSortedBag for null");
          }
          return new UnmodifiableSortedBag<E>(bag);
      }
docs: This method will take a MutableSortedBag and wrap it directly in a UnmodifiableSortedBag.