proposition: Lemma coset_mulP : associative coset_mul.
proof: Proof. by move=> B C D; apply: val_inj; rewrite /= mulgA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)