proposition: Lemma card_lcosets G H : #|lcosets H G| = #|G : H|.
proof: Proof. by rewrite -card_invg invg_lcosets !invGid. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)