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

Lemma wneg_wordToN: forall sz (w: word sz), wordToN w <> 0%N -> wordToN (wneg w) = (Npow2 sz - wordToN w)%N.