So a coroutine is basically something you can explicitly yield control flow from or give control flow to. neco doesn't really let you do that. Caller of neco_resume isn't relinquishing the control flow to another coroutine. neco_yield looks like a coroutine yield, but for asymmetric coroutine it should yield to caller, for symmetric coroutine it should let you specify who you are yielding to. This one does neither.

And I feel if you stretch the definition of coroutine to include this, then you can basically call most multitasking systems coroutines, rendering the concept mostly useless.