proposition: Lemma normsGI A B G : A \subset 'N(B) -> G :&: A \subset 'N(G :&: B).
proof: Proof. by move=> nBA; rewrite !(setIC G) normsIG. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)