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

Lemma ZToWord_minus: forall sz a b, ZToWord sz (a - b) = ZToWord sz a ^- ZToWord sz b.