getElem.lean:2:2-2:6: error: failed to prove index is valid, possible solutions:
  - Use `have`-expressions to prove the index is valid
  - Use `a[i]!` notation instead, runtime check is perfomed, and 'Panic' error message is produced if index is not valid
  - Use `a[i]?` notation instead, result is an `Option` type
  - Use `a[i]'h` notation instead, where `h` is a proof that index is valid
a : Array Nat
i : Nat
⊢ i < Array.size a
def f2 : (a : Array Nat) → Fin (Array.size a) → Nat :=
fun a i => a[i]
def f3 : {n : Nat} → (a : Array Nat) → n ≤ Array.size a → Fin n → Nat :=
fun {n} a h i => a[i]
def f5 : (i : Nat) → i < n → Nat :=
fun i h => a[i]'(_ : i < Array.size a)
def f6 : Nat → Nat :=
fun i => a[i]!
def f7 : Nat → Option Nat :=
fun i => a[i]?