code: public void subscribe(Class<? extends Event> eventType, Class<? extends Subscriber> subscriberType) {
          if (!subscriptionMap.containsKey(eventType)) {
              subscriptionMap.put(eventType, new ArrayList<>());
          }
          final ArrayList<Class<? extends Subscriber>> subscribers = subscriptionMap.get(eventType);
          if (!subscribers.contains(subscriberType)) {
              subscribers.add(subscriberType);
          }
      }
docs: {@inheritDoc}
  @since 1.0.0