∀ {α : Type u_2} {β : Type u_1} {b : β} {f : α → β → β} (head : α) (tail : List α),
  List.scanr f b (head :: tail) ≠ [] →
    -1 * ↑(List.scanr f b tail).length + 1 ≤ 0 → (List.scanr f b tail).length - 1 < (List.scanr f b tail).length