Ltac wlt_ind :=
  match goal with
  | [ |- forall (n: word ?len), ?P ] =>
    refine (well_founded_ind (@wlt_wf len) (fun n => P) _)
  end.

Lemma wordToZ_ZToWord: forall z sz, (- Z.of_nat (pow2 sz) <= z < Z.of_nat (pow2 sz))%Z -> wordToZ (ZToWord (S sz) z) = z.