proposition: Lemma astabsJ A : 'N(A | 'J) = 'N(A).
proof: Proof. by apply/setP=> x; rewrite -2!groupV !inE -conjg_preim -sub_conjg. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)