. intro i hi h_ind acc
  unfold Array.ofFn.go
  rw [dif_pos (Nat.succ_lt_succ hi), dif_pos hi]
  rw [h_ind]
  rfl
case refine_2
α✝ α : Type u
n : Nat
f : Fin (Nat.succ n) → α
i : Nat
acc : Array α
hlt : i < n
⊢ ∀ (k : Nat),
    k < n →
      (∀ (acc : Array α), ofFn.go f (Nat.succ (Nat.succ k)) acc = ofFn.go (f ∘ Fin.succ) (Nat.succ k) acc) →
        ∀ (acc : Array α), ofFn.go f (Nat.succ k) acc = ofFn.go (f ∘ Fin.succ) k acc
no goals