* for APIs like {@link #whenAllComplete whenAllComplete(...)}{@code .}{@link
   * FutureCombiner#call(Callable, Executor) call(...)}, where it is easy to use a new input from
   * the {@code call} implementation but forget to add it to the arguments of {@code
   * whenAllComplete}.
   *
   * <p>If you are looking for a method to determine whether a given {@code Future} is done, use the
   * instance method {@link Future#isDone()}.

   * stable. There is usually no reason to retain a reference of type {@code SetView}; typically,
   * you either use it as a plain {@link Set}, or immediately invoke {@link #immutableCopy} or
   * {@link #copyInto} and forget the {@code SetView} itself.
   *
   * @since 2.0
   */
  public abstract static class SetView<E extends @Nullable Object> extends AbstractSet<E> {
    private SetView() {} // no subclasses but our own

    /**