> A Subscriber should be dead simple to implement, and it really is.

Again, I quote the same sentence from below: "The Reactive Streams API prescribes that all processing of elements (onNext) or termination signals (onError, onComplete) happens outside of the execution stack of the Publisher." 

Maybe there's still a basic misunderstanding about how this rule is meant. Our previous understanding was that all the asynchronous handoff happened **inside** the `onX` methods (and the same for `request`/`cancel`). So, it  previously meant that the caller of the `onX` method never has to care for scheduling but it's always the responsibility of the callee to ensure proper scheduling. Has this changed? 

In this previous sense, I wouldn't call it "dead simple" to implement a `Subscriber` in a way that complies with this rule.