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 sz w, exists k, wordToZ (ZToWord sz w) = (w - k * Z.of_N (Npow2 sz))%Z.