proposition: Lemma normCs A : 'N(~: A) = 'N(A).
proof: Proof. by apply/setP=> x; rewrite -groupV !inE conjCg setCS sub_conjg. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)