proposition: Lemma commrX x y n : comm x y -> comm x (y ^+ n).
proof: Proof. rewrite /comm => com_xy. by elim: n => [|n IHn]; rewrite ?commr1 // exprS commrM. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)