∀ {α : Type u_1} (x : List α)
  (f :
    List.below (motive := fun x => ∀ (x_1 x_2 : ℕ), (x.zipIdx x_1)[x_2]? = Option.map (fun a => (a, x_1 + x_2)) x[x_2]?)
      x)
  (x_1 x_2 : ℕ), (x.zipIdx x_1)[x_2]? = Option.map (fun a => (a, x_1 + x_2)) x[x_2]?