proposition: Lemma sub_cent1 A x : (A \subset 'C[x]) = (x \in 'C(A)).
proof: Proof. by rewrite -cent_cycle centsC cycle_subG. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)