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