proposition: Lemma tofracB : {morph tofrac: x y / x - y}.
proof: Proof. exact: rmorphB. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(ssrAC choice tuple bigop ssralg poly polydiv) mathcomp(generic_quotient)