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

Lemma sext_natToWord': forall sz1 sz2 n, (2 * n < pow2 sz1)%nat -> sext (natToWord sz1 n) sz2 = natToWord (sz1 + sz2) n.