proposition: Lemma astab_normal S : 'C(S | to) <| 'N(S | to).
proof: Proof. by rewrite /normal astab_sub astab_norm. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)