code: public static Optional<? extends LinkedIOSubchannel> downstreamChannel(
              Manager hub, IOSubchannel upstreamChannel) {
          return upstreamChannel.associated(
              new KeyWrapper(hub), LinkedIOSubchannel.class);
      }
docs: Returns the linked downstream channel that has been created for the
  given component and (upstream) subchannel. If more than one linked
  subchannel has been created for a given component and subchannel,
  the linked subchannel created last is returned.
  
  @param hub the component that manages this channel
  @param upstreamChannel the (upstream) channel
  @return the linked downstream subchannel created for the
  given component and (upstream) subchannel if it exists