docs: @Handler(channels = EncryptedChannel.class)
      public void onClosed(Closed event, IOSubchannel encryptedChannel)
              throws SSLException, InterruptedException {
          @SuppressWarnings("unchecked")
          final Optional<PlainChannel> plainChannel
              = (Optional<PlainChannel>) LinkedIOSubchannel
                  .downstreamChannel(this, encryptedChannel);
          if (plainChannel.isPresent()) {
              plainChannel.get().upstreamClosed();
          }
      }