repeat rewrite wordToN_nat; repeat rewrite NToWord_nat.
rewrite nat_of_Nplus.
repeat rewrite nat_of_N_of_nat.
reflexivity.
Qed.

(* Specifications: *)
Theorem wmult_alt : forall sz (x y : word sz), wmult x y = wmultN x y.
unfold wmultN, wmult, wordBinN, wordBin; intros.