code: @Override
    public synchronized void close() throws IOException {
      if (opened) {
        face.removeRegisteredPrefix(registrationId);
        contentStore.clear();
        announcementService.announceExit(publisherId);
      }
    }
docs: TODO this should not clear content store or remove registered prefix; do that in the future to allow subscribers
  to retrieve still-alive messages
  
  @throws IOException if the group exit announcement fails