code: public void tellSubscribers( Actor sender, String topic, Object message) {
          List<ReceiverActor> subscriber = topic2Subscriber.get(topic);
          if ( subscriber != null ) {
              subscriber.stream()
                  .filter(subs -> !subs.equals(sender)) // do not receive self sent
                  .forEach(subs -> subs.receiveTell(topic, message) );
          }
      }
docs: send a fire and forget message to all
  
  @param sender
  @param topic
  @param message