[webjprgm]

Correct me if I'm wrong, but in a formal sense you get the synchronous system inside the asynchronous one trivially. So the two are of equivalent power.

If you imagine that in the case of threading systems, you can implement a multi-threaded system with your own green threads library, which is async on top of sync. You can get sync on top of async with native threads by simply using one thread. Or, if you want to be complicated, by using multiple threads with locks that serialize to a single synchronous ordering.