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 Zeqb_true_ZToWord: forall {sz: nat} (x y: Z),
(x =? y)%Z = true ->
ZToWord sz x = ZToWord sz y.