Goal:
  
  A : Type
  P : nat → A → Type
  i : nat
  x : A
  n : nat
  hd : A
  tl : list A
  p : P n hd
  h : Alli P (S n) tl
  e : P (S n + i) x
  IHh : ∀ (i : nat) (x : A), nth_error tl i = Some x → P (S n + i) x
  ============================
   (S n + i = n + S i)


DEBUG:Going to execute:
intros; Zify.zify_pre_hook; zify_elim_let; zify_op;
            Zify.zify_iter_specs