public NavigableMap<K, V> descendingMap() {
      return asMap(set.descendingSet(), function);
    }
  }

  private static <E extends @Nullable Object> Set<E> removeOnlySet(Set<E> set) {
    return new ForwardingSet<E>() {
      @Override
      protected Set<E> delegate() {
        return set;
      }

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