unfold foldl.loop
case zero.refl
α : Sort u_1
n : ℕ
f : α → Fin n → α
init acc : α
hj : n + 0 = n
⊢ foldl.loop n f acc n = foldl'.aux n init (fun i h acc => f acc ⟨i, h⟩) n ⋯ acc
case zero.refl
α : Sort u_1
n : ℕ
f : α → Fin n → α
init acc : α
hj : n + 0 = n
⊢ (if h : n < n then foldl.loop n f (f acc ⟨n, h⟩) (n + 1) else acc) =
    foldl'.aux n init (fun i h acc => f acc ⟨i, h⟩) n ⋯ acc