docs: public Collection<Registration> getRegistrations(String topic, boolean forceCreate) {
          Collection<Registration> listenerList = registrations.get(topic);
          if (listenerList == null && forceCreate) {
              ConstructorFunction<String, Collection<Registration>> func
                      = key -> newSetFromMap(new ConcurrentHashMap<Registration, Boolean>());
              return ConcurrencyUtil.getOrPutIfAbsent(registrations, topic, func);
          }
          return listenerList;
      }