proposition: Lemma normsU : A \subset 'N(B :|: C).
proof: Proof. by apply/normsP=> x Ax; rewrite conjUg !(normsP _ x Ax). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)