proposition: Lemma comm_mxM f g g' : comm_mx f g -> comm_mx f g' -> comm_mx f (g *m g').
proof: Proof. by rewrite /comm_mx mulmxA => ->; rewrite -!mulmxA => ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg countalg finalg zmodp)