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

Lemma wminus_wordToZ': forall sz (w1 w2: word (sz + 1)), existT word _ (w2 ^- w1) <> existT word _ (wpow2 sz) -> wordToZ (w1 ^- w2) = (- wordToZ (w2 ^- w1))%Z.