Hi Mark,
since you are building a concurrent system based on messaging (Actors),
you can never safely know that a queue is really and finally empty *just* by
looking at the queue.
Reason: there might be a message in flight to this Actor, and the second
after you've inspected the queue to be empty
I am trying to find a way to find out when all children actors are finished
working (processed all the messages in their respective queues). Think
about graph traversal when each vertex discovery requires its own actor.
The depth of the tree is finite and known and each level is requires its
