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