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


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