code: public <E> Collection<E> unmodifiable(Collection<E> input)
      {
          if (input == null) {
              return null;
          }
          return Collections.unmodifiableCollection(input);
      }
docs: Returns an unmodifiable view of the specified collection.
  
  @param <E> the type of the elements in the collection
  @param input the collection to wrap in an unmodifiable bridge
  @return an unmodifiable view of the collection