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

Lemma wminus_Z: forall sz (a b : word sz), a ^- b = ZToWord sz (wordToZ a - wordToZ b).