code: @Handler(channels = EncryptedChannel.class)
      public void onPurge(Purge event, IOSubchannel encryptedChannel) {
          @SuppressWarnings("unchecked")
          final Optional<PlainChannel> plainChannel
              = (Optional<PlainChannel>) LinkedIOSubchannel
                  .downstreamChannel(this, encryptedChannel);
          if (plainChannel.isPresent()) {
              plainChannel.get().purge();
          }
      }
docs: Forwards a {@link Purge} event downstream.
  
  @param event the event
  @param encryptedChannel the encrypted channel