simp at hj
case zero
α : Sort u_1
n : ℕ
f : α → Fin n → α
init : α
i : ℕ
hj : i + 0 = n
acc : α
⊢ foldl.loop n f acc i = foldl'.aux n init (fun i h acc => f acc ⟨i, h⟩) i ⋯ acc
case zero
α : Sort u_1
n : ℕ
f : α → Fin n → α
init : α
i : ℕ
hj✝ : i + 0 = n
acc : α
hj : i = n
⊢ foldl.loop n f acc i = foldl'.aux n init (fun i h acc => f acc ⟨i, h⟩) i ⋯ acc