docs: protected void retry(final Notification not) {
          if (!pendings.contains(not.subscriber.getCallback())) {
              // We don't have a current retry for this callback pending, so we
              // will schedule the retry
              pendings.add(not.subscriber.getCallback());
              timer.schedule(new TimerTask() {
                  @Override
                  public void run() {
                      pendings.remove(not.subscriber.getCallback());
                      enqueueNotification(not);
                  }
              }, TWO_MINUTES);
          } else {
              // There is a retry in front of this one, so we will just schedule
              // it to retry again in a bit
              timer.schedule(new TimerTask() {
                  @Override
                  public void run() {
                      retry(not);
                  }
              }, TWO_MINUTES);
          }
      }