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