code: public synchronized boolean addIncomingChatMarkerMessageListener(ChatMarkersListener listener) {
          boolean res = incomingListeners.add(listener);
          if (!enabled) {
              serviceDiscoveryManager.addFeature(ChatMarkersElements.NAMESPACE);
              enabled = true;
          }
          return res;
      }
docs: Register a ChatMarkersListener. That listener will be informed about new
  incoming markable messages.
  
  @param listener ChatMarkersListener
  @return true, if the listener was not registered before