repeat rewrite wordToN_nat; repeat rewrite NToWord_nat.
rewrite nat_of_Nminus.
do 2 f_equal.
apply Npow2_nat.
apply nat_of_N_of_nat.
Qed.

(* Specifications: *)
Theorem wminus_Alt : forall sz (x y : word sz), wminus x y = wminusN x y.
intros; unfold wminusN, wminus; rewrite wneg_alt; apply wplus_alt.