intro i j hj
α : Sort u_1
n : ℕ
f : α → Fin n → α
init : α
⊢ ∀ (i j : ℕ) (hj : i + j = n) (acc : α), foldl.loop n f acc i = foldl'.aux n init (fun i h acc => f acc ⟨i, h⟩) i ⋯ acc
α : Sort u_1
n : ℕ
f : α → Fin n → α
init : α
i j : ℕ
hj : i + j = n
⊢ ∀ (acc : α), foldl.loop n f acc i = foldl'.aux n init (fun i h acc => f acc ⟨i, h⟩) i ⋯ acc