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

Lemma wmsb_zext: forall sz (w: word sz) b n, n <> 0 -> wmsb (zext w n) b = false.