Join Continuation
Provides a mechanism for synchronizing
the processing of a set of messages.
Set of results is sent along as a token.
Example:
Actor[] actors = { searcher0, searcher1,
        searcher2, searcher3 };
join( actors<-find( phrase ) ) @
   resultActor<-output( token );
Send the find( phrase ) message to each actor in
actors[] then after all have completed send the result to
resultActor with an output( … ) message.