Stackless coroutines can call other functions just fine (they would completely unusable otherwise). What they can't do is delegate suspension to any called function, i.e. any called function must return before the coroutine can yield as exactly one activation frame (the top level) can be suspended.

That's just a simple example. I think the following should work, possibly with modification for forwarding:

    class Stooge {...};

    CoroutineIterator<Stooge> three_stooges() {
       co_yield Stooge("Larry");
       co_yield Stooge("Curly");
       co_yield Stooge("Moe");
    }