proposition: Lemma tofrac_is_additive: additive tofrac.
proof: Proof. move=> p q /=; unlock tofrac. rewrite -[X in _ = _ + X]pi_opp -[RHS]pi_add. by rewrite /addf /oppf /= !numden_Ratio ?(oner_neq0, mul1r, mulr1). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(ssrAC choice tuple bigop ssralg poly polydiv) mathcomp(generic_quotient)