proposition: Lemma LagrangeMr G H : (#|G : H| * #|H|)%N = #|G * H|.
proof: Proof. by rewrite mulnC LagrangeMl -card_invg invMg !invGid. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)