proposition: Lemma commrD x y z : comm x y -> comm x z -> comm x (y + z).
proof: Proof. by rewrite /comm mulrDl mulrDr => -> ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)