code: public int getNumberOfDependents() {
      int count = 0;
      for (Completion p = stack; p != null; p = p.next)
        ++count;
      return count;
    }
docs: Returns the estimated number of CompletableFutures whose completions are awaiting completion of
  this CompletableFuture. This method is designed for use in monitoring system state, not for
  synchronization control.
  
  @return the number of dependent CompletableFutures