Goal:
    A : Type  f : nat → A  n : nat  Hn : n < 0
  ============================
   (nth_error (unfold 0 f) n = Some (f n))


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