proposition: Lemma comm_mx_sym f g : comm_mx f g -> comm_mx g f.
proof: Proof. by rewrite /comm_mx. 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)