proposition: Lemma QSs x y: ratp x -> ratp y -> ratp (x +q y).
proof: Proof. move => /BQ_P [_ nxz dxp _] /BQ_P [_ nyz dyp _]. move: (ZpsS_prod dxp dyp); apply: BQ_of_pair_prop4. by apply:ZSs; apply:ZSp => //; apply: BZps_sBZ. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)