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

Lemma combine_zero: forall n m, combine (natToWord n 0) (natToWord m 0) = natToWord _ 0.