Local Lemma wmultZ_helper: forall a b k1 k2 p,
((a - k1 * p) * (b - k2 * p) = a * b - (k1 * b + k2 * a - k1 * k2 * p) * p)%Z.

Lemma ZToWord_mult: forall sz a b, ZToWord sz (a * b) = ZToWord sz a ^* ZToWord sz b.