futures.add(submitAndAddQueueListener(executorService, it.next(), futureQueue));
      --ntasks;
      int active = 1;

      while (true) {
        Future<T> f = futureQueue.poll();
        if (f == null) {
          if (ntasks > 0) {
            --ntasks;
            futures.add(submitAndAddQueueListener(executorService, it.next(), futureQueue));
            ++active;