∀ {α : Type u_1} {r : α → α → Prop} {a : α} {b : List α},
  Acc r a →
    Acc (List.Shortlex r) b →
      (∀ (s : List α), s.length < (a :: b).length → Acc (List.Shortlex r) s) → Acc (List.Shortlex r) (a :: b)