[derkha]

Well, that change would also break the proof in general :) . But I see your point.

I've written about overflow checking some more below, but what you'd really want for that is some solid support of subtypes or refinement types - it's not just integers that have to become bounded, but also most data structures. Lean isn't quite there yet, unfortunately, but this should change in the near future with its new focus on powerful automation.