proposition: Lemma cardMg_divn G H : #|G * H| = (#|G| * #|H|) %/ #|G :&: H|.
proof: Proof. by rewrite mul_cardG mulnK ?cardG_gt0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)