It would be nice if we had a way to query whether a thread has joined the Suspendible Thread set (STS) or not. Right now, when a thread calls STS::join() it just increments a counter in the STS object so there is not way to know later whether that thread is still in the STS or has left it. If we had a way to determine that we could add asserts to:

- make sure that a thread does not join the STS twice
- check that a thread has joined the STS before calling a method which assumes that

A straightforward way to do this is to add a flag in our thread data structure (maybe only in non-product builds?) which is set and unset as the thread joins and leaves the STS.