> The mechanism was that it had threads and coroutines, a function needing to make use of the C API in a seamless manner would simply run as a thread rather than a coroutine.

This reintroduces the colored functions that we were trying to get away from in the first place, by adopting stackful coroutines/fibers. Why not use async at that point? I can understand that Alef didn't, because stackless mechanisms were not well understood at the time. But it's plausible that we can do better.