abstract class Channel[T] = { def !(msg: T): Unit }

 def channel: Channel = {
   if (senderFuture.isDefined) {
     new Channel[Any] {
       val future = senderFuture.get
       def !(msg: Any) = future completeWithResult msg
     }
   } else if (sender.isDefined) {
     new Channel[Any] {
       val client = sender
       def !(msg: Any) = client ! msg
     }
   } else IllegalActorStateException("No channel available")
 }

So the server does, say

replyTo = channel

(or put it in a queue, or whatever) and then later uses it like this

replyTo ! response

just like sender would be used in a direct reply in fire-and-forget
mode.

"replyTarget ! response" would be equivalent to "reply(response)"
(the implementation of replyTarget is, of course, very similar to that
of reply).

MsgTarget is identical to Lift's SimpleActor - maybe better called
SimpleActorRef or so?