proposition: Lemma normsG A G : A \subset G -> A \subset 'N(G).
proof: Proof. by move=> sAG; apply: subset_trans (normG G). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)