proposition: Lemma pi_opp : {morph \pi : x / oppf x >-> opp x}.
proof: Proof. move=> x; unlock opp; apply/eqmodP; rewrite /= /equivf /oppf /=. by rewrite !numden_Ratio ?(domP,mulf_neq0) // mulNr mulrN -equivf_r. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(ssrAC choice tuple bigop ssralg poly polydiv) mathcomp(generic_quotient)