simp only [Array.range, Nat.fold, flip, Array.get_push]
case inr
x n : Nat
ih : ∀ (h : x < n), (Array.range n)[x] = x
h : x < n + 1
x_eq_n : x = n
⊢ (Array.range (n + 1))[x] = x
case inr
x n : Nat
ih : ∀ (h : x < n), (Array.range n)[x] = x
h : x < n + 1
x_eq_n : x = n
⊢ (if h : x < (Nat.fold (flip Array.push) n (Array.mkEmpty (n + 1))).size then
      (Nat.fold (flip Array.push) n (Array.mkEmpty (n + 1)))[x]
    else n) =
    x