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

Lemma extz_is_mult_pow2: forall sz n d, extz (natToWord sz n) d = natToWord (d + sz) (pow2 d * n).