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

Lemma wminus_wminusZ: forall (sz : nat) (w1 w2 : word sz), w1 ^- w2 = wminusZ w1 w2.