Wip sip23 √

> That statement is fortunately false—if you pass around a Twitter EC, you are in full control over capturing at every ec.execute as per my CapturingEC.

My point is that I can't use Twitter's EC to affect local propagation in 3rd party libraries. For example, propagation would fail if I use a Twitter EC with `scala.concurrent.Promise`. Put another way, propagation should be a function of the EC (or, as I outlined above, maybe another type of context), not of the uses of ECs.