proposition: Lemma in_alg_comm : commr_rmorph (in_alg A) a.
proof: Proof. move=> r /=; by rewrite /GRing.comm comm_alg. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)