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