Goal:
  
  A : Type
  n : nat
  a : A
  l : list A
  l' : list A
  IHl : ∀ (n : nat) (l' : list A),
          n ≤ #|l| → skipn n (l ++ l') = skipn n l ++ l'
  Hn : S n ≤ S #|l|
  ============================
   (n ≤ #|l|)


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