code: boolean unregisterTopicSessions(String topic, Collection<Session> sessions) {
  		boolean unregister = false;
  		if (sessions != null && !sessions.isEmpty()) {
  			Collection<Session> all = map.get(topic);
  			if(all != null) {
  				unregister = all.removeAll(sessions);
  				if (all.isEmpty()) {
  					map.remove(topic);
  				}
  			}
  		}
  		return unregister;
  	}
docs: Unregister sessions for topic
  
  @param topic
  @param sessions
  @return