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