proposition: Lemma mulC : commutative mul.
proof: Proof. elim/quotW=> x; elim/quotW=> y; rewrite !piE /mulf. by rewrite [_ * (\d_x)]mulrC [_ * (\n_x)]mulrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(ssrAC choice tuple bigop ssralg poly polydiv) mathcomp(generic_quotient)