I think "happens outside of the execution stack of the Publisher" needs to be deleted or changed.

The only rule that a `Subscriber` should have is "don't block".

If it is doing synchronous transformations/filtering/etc, that's fine. If it's blocking a thread on IO, that's not fine.

I should be able to write code like this:

```java
void onNext(T t) {
  queue.offer(transform(t));
}
```