In effect, this example emulates simple tasklets as are used in Stackless Python, as long as you use a yield expression to invoke routines that would otherwise "block".However, what happens if you can't use a yield expression to invoke routines that would otherwise block?
For instance, say coro() is a coroutine that calls third_party_1() that calls third_party_2() that calls blocker() where blocker() blocks. If third_party_1() and third_party_2() are intermediary functions that exist in third-party code, they won't know how to "do the right" thing using yield. If third_party_1() and third_party_2() don't do the right thing with yield, there's no way for blocker() to yield back to the scheduler.
Ideally, I'd like for coro() to be a coroutine that blocks within blocker(), leaving third_party_1() and third_party_2() ignorant of the whole process. If you know how, please tell me!