intros sz a b c H.
apply (f_equal (fun x => x ^+ ^~ c)) in H.
repeat rewrite <- wplus_assoc in H.
rewrite wminus_inv in H.
repeat rewrite (wplus_comm _ (wzero sz)) in H.
repeat rewrite wplus_unit in H.
assumption.
Qed.

Lemma wlt_lt': forall sz a b, (a < pow2 sz)%nat
-> natToWord sz a < b
-> (wordToNat (natToWord sz a) < wordToNat b)%nat.