throw new UnsupportedOperationException();
      }
    };
  }

  private static <E extends @Nullable Object> SortedSet<E> removeOnlySortedSet(SortedSet<E> set) {
    return new ForwardingSortedSet<E>() {
      @Override
      protected SortedSet<E> delegate() {
        return set;
      }

      @Override
      public boolean add(@ParametricNullness E element) {