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

Local Lemma wminusZ_helper: forall a b k1 k2 p, ((a - k1 * p) - (b - k2 * p) = a - b - (k1 - k2) * p)%Z.