Goal:
    A : Type  n : nat  l' : list A  Hn : S n ≤ 0
  ============================
   (skipn (S n) l' = skipn (S n) [] ++ l')


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