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 (S sz)), w2 ^- w1 <> wpow2 sz -> wordToZ (w1 ^- w2) = (- wordToZ (w2 ^- w1))%Z.