proposition: Lemma subgacentE : 'C_(S | to)(A) = 'Fix_(S | to)(A).
proof: Proof. by rewrite gacentE setIA (setIidPl sSR). Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)