Qed.
Hint Rewrite roundTrip_0 : wordToNat.

(* Specifications: *)
Lemma wordToNat_natToWord' : forall sz w, exists k, wordToNat (natToWord sz w) + k * pow2 sz = w.
induction sz as [|sz IHsz]; simpl; intro w; intuition; repeat rewrite untimes2.