code: public boolean addRegistration(String topic, Registration registration) {
          Collection<Registration> registrations = getRegistrations(topic, true);
          if (registrations.add(registration)) {
              registrationIdMap.put(registration.getId(), registration);
              pingNotifiableEventListener(topic, registration, true);
              return true;
          }
          return false;
      }
docs: Adds a registration for the {@code topic} and notifies the listener and service of the listener
  registration. Returns if the registration was added. The registration might not be added
  if an equal instance is already registered.
  
  @param topic        the event topic
  @param registration the listener registration
  @return if the registration is added