proposition: Lemma commrM x y z : comm x y -> comm x z -> comm x (y * z).
proof: Proof. by move=> com_xy; rewrite /comm mulrA com_xy -!mulrA => ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)