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

Lemma wlshift_combine_extz: forall sn sl (wl: word sl) ssu (wu: word (ssu + sn)), existT word (sl + (ssu + sn)) (wlshift (combine wl wu) sn) = existT word (sn + (sl + ssu)) (extz (combine wl (split1 ssu _ wu)) sn).