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 pow2_minus_one_wones: forall {sz} (w: word sz),
wordToNat w = pow2 sz - 1 -> w = wones sz.