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

Lemma wordToNat_combine: forall sz1 (w1: word sz1) sz2 (w2: word sz2), wordToNat (combine w1 w2) = wordToNat w1 + pow2 sz1 * wordToNat w2.