Going by Yury’s tests and implementation, I think it is intended that:

* An “async def” coroutine factory function is accepted and passed through unmodified
* An generator–coroutine factory function that has already been transformed is also accepted
* If a wrapped function (not a normal coroutine nor generator factory) returns a coroutine instance (e.g. if the function was already decorated), the return value is never wrapped a second time

I will have a go at writing this up.