wf2.lean:3:8-3:17: error: failed to prove termination, possible solutions:
  - Use `have`-expressions to prove the remaining goals
  - Use `termination_by` to specify a different well-founded relation
  - Use `decreasing_by` to specify your own tactic for discharging this kind of goal
y x : Nat
h✝ : x < y
⊢ x - 1 < x