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

Lemma wneg_wordToZ: forall sz (w: word (S sz)), w <> wpow2 sz -> wordToZ (wneg w) = (- wordToZ w)%Z.