Suppose you want to look around in the caller's frame. You try 13!:19'', but it restarts immediately, calls problem again, and fails.
That seems like a valid use case. More generally, it seems like it would be useful to poke around in _any_ stack frame, without destroying the ones beneath (or on top of, depending on which way your stacks grow) it. But that leads to forking stacks and continuations.
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