code: @SuppressWarnings("checkstyle:hiddenfield")
    public <T, U extends T> void register(final Class<T> type, final Set<EventHandler<U>> handlers) {
      this.handlers.put(type, new ExceptionHandlingEventHandler<>(
          new BroadCastEventHandler<>(handlers), this.errorHandler));
    }
docs: Register a new event handler.
  
  @param type     Message type to process with this handler.
  @param handlers A set of handlers that process that type of message.
  @param <T>      Message type.
  @param <U>      Type of message that event handler supports. Must be a subclass of T.