∀ {β : Type u_1} {α : Type u_2} {i : ℕ} {f : β → α → β} (init : β) (as : Array α),
  -1 * ↑as.size + 1 ≤ 0 →
    i + 1 = (Array.scanl f init as).size - 1 →
      i = as.size - 1 →
        ∀ (w : ℕ),
          w + 1 ≤ ((Array.scanl f init as).extract 0 (i + 1)).size → w < (Array.scanl f init (as.extract 0 i)).size