That's one nice thing provable in Haskell's type/effect system, though, that the ambient effects available are no stronger than X for various descriptions of X.

That's one nice thing provable in Haskell's type/effect system, though, that the ambient effects available are no stronger than X for various descriptions of X. I know you dislike that whole regime, but it at least in this specific case explicitly circumscribes situations where, e.g., Cont is too strong.