proposition: Lemma mulrnBl n : {morph (fun x => x *+ n) : x y / x - y}.
proof: Proof. move=> x y; elim: n => [|n IHn]; rewrite ?subr0 // !mulrS -!addrA; congr(_ + _). by rewrite addrC IHn -!addrA opprD [_ - y]addrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)