code: @Override
  	public Collection<String> removeSessionsToTopic(Collection<Session> sessions) {
  		if (sessions != null && !sessions.isEmpty()) {
  			Collection<String> topicUpdated = new ArrayList<>();
  			for (String topic : map.keySet()) {
  				if(unregisterTopicSessions(topic, sessions)) {
  					topicUpdated.add(topic);
  					sendSubscriptionEvent(Constants.Topic.SUBSCRIBERS + Constants.Topic.COLON + topic, getNumberSubscribers(topic));
  				}
  			}
  			return topicUpdated;
  		}
  		return Collections.EMPTY_LIST;
  	}
docs: Remove sessions cause they are closed
  
  @param sessions