Qed.

Lemma mod2p : forall a b c', (a * 2 ^ w + b) mod 2 ^ p = (c' * 2 ^ w + b) mod 2 ^ p.
intros a b c'. replace w with (w - p + p). rewrite Zpower_exp. do 2 rewrite Zmult_assoc. do 2 rewrite Zplus_comm with (m := b).