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

Lemma sext_sext: forall sz (w: word sz) n1 n2, existT word _ (sext w (n1 + n2)) = existT word _ (sext (sext w n1) n2).