*
   * <p>If the supplied future is {@linkplain #isDone done} when this method is called and the call
   * is accepted, then this future is guaranteed to have been completed with the supplied future by
   * the time this method returns. If the supplied future is not done and the call is accepted, then
   * the future will be <i>set asynchronously</i>. Note that such a result, though not yet known,