docs: @Handler(channels = EncryptedChannel.class)
      public void onIOError(IOError event, IOSubchannel encryptedChannel)
              throws SSLException, InterruptedException {
          @SuppressWarnings("unchecked")
          final Optional<PlainChannel> plainChannel
              = (Optional<PlainChannel>) LinkedIOSubchannel
                  .downstreamChannel(this, encryptedChannel);
          plainChannel.ifPresent(channel -> fire(new IOError(event), channel));
      }