generalize (div2_double (pow2 sz)); simpl; intro Hr; rewrite Hr; clear Hr.
f_equal.
generalize (mod2_double (pow2 sz)); auto.
auto.
Qed.

(* Specifications: *)
Theorem wminus_inv : forall sz (x : word sz), x ^+ ^~ x = wzero sz.
intros sz x; rewrite wneg_alt; rewrite wplus_alt; unfold wnegN, wplusN, wzero, wordBinN; intros.