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

Lemma wordToZ_inj: forall sz (w1 w2: word sz), wordToZ w1 = wordToZ w2 -> w1 = w2.