proposition: Lemma polyOver_mulr_2closed : GRing.mulr_2closed (polyOver S).
proof: Proof. move=> p q /polyOverP Sp /polyOverP Sq; apply/polyOverP=> i. by rewrite coefM rpred_sum // => j _; rewrite rpredM. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)