letRecMissingAnnotation.lean:4:6-4:34: 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
as : Array Nat
i s : Nat
h : i < Array.size as
⊢ Array.size as - (i + 2) < Array.size as - i