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

Lemma sext_wneg_natToWord: forall sz2 sz1 sz n (e: sz1 + sz2 = sz), (2 * n < pow2 sz1)%nat -> eq_rect (sz1 + sz2) word (sext (wneg (natToWord sz1 n)) sz2) sz e = wneg (natToWord sz n).