proposition: Lemma astab_ract S : 'C(S | to \ sAD) = 'C_A(S | to).
proof: Proof. by rewrite setIA (setIidPl sAD). Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)