code: public Bus<M> addObserver(BusObserver<M> observer) {
  		if(observer != null) {
  			logger.info("adding observer of class '{}'", observer.getClass().getSimpleName());
  			observers.add(observer);
  		}
  		return this;
  	}
docs: Adds an observer to this message bus.
  
  @param observer
  the observer.
  @return
  the object itself, for method chaining.