Cute. You probably can shorten the argument a bit by accepting Rice's theorem ( https://en.wikipedia.org/wiki/Rice's_theorem https://en.wikipedia.org/wiki/Rice

Cute. You probably can shorten the argument a bit by accepting Rice's theorem ( https://en.wikipedia.org/wiki/Rice's_theorem https://en.wikipedia.org/wiki/Rice's_theorem ) instead of explicitly building the Rice widget (which is pretty much how Rice's theorem is proven).