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

Lemma wrshifta_extz_sext: (* Note: not axiom free *) forall sz (w: word sz) n1 n2, existT word _ (wrshifta (extz w (n1 + n2)) n1) = existT word _ (sext (extz w n2) n1).