proposition: Lemma commrMn x y n : comm x y -> comm x (y *+ n).
proof: Proof. rewrite /comm => com_xy. by elim: n => [|n IHn]; rewrite ?commr0 // mulrS commrD. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)