Goal:
    G : t  l : labelling  Hl : correct_labelling l  x : V.t
  ============================
   (Z.of_nat (l x) + 0 <= Z.of_nat (l x))


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