Lean Kernel Arena / bugs/large-elim-param

Proof of False via incorrect large elimination restriction.

If the check for whether a level is surely not zero is implemented wrong, in particular if it incorrectly returns true for params, we can create a universe-polymorphic

inductive MyBool.{u} : Sort u | tt | ff

where the recursor MyBool.rec.{1,0} can do large elimination of a Prop. Because of proof irrelevance we have tt = ff, so we can derive a contradiction.

Found by Anthony Wang using Aristotle, breaking the mini checker for the T-shirt bounty; it was fixed the same day.