abstract ImmutableSortedSet<E> createDescendingSet();

  /** @since 12.0 */
  @GwtIncompatible // NavigableSet
  @Override
  public abstract UnmodifiableIterator<E> descendingIterator();

  /** Returns the position of an element within the set, or -1 if not present. */
  abstract int indexOf(@CheckForNull Object target);

  /*
   * This class is used to serialize all ImmutableSortedSet instances,