As for ordered cancellations, my only point was that having five nested blocks is ugly. That may seem to be no big deal, but the goal here is really to get peop

As for ordered cancellations, my only point was that having five nested blocks is ugly. That may seem to be no big deal, but the goal here is really to get people think in structured way. If the syntactic constructs get into there way (the identation gets my code beyond the right edge of the screen) they are likely to cut corners. E.g. use a single scope. Which could in turn cause problems during shutdown. As for producer/consumer I think you want to close the producer first, consumer second, rather than do it in undefined order (in parallel). That way you can be sure that there are no unconsumed messages before you shut down.