proposition: Lemma subgacent1E : 'C_(S | to)[a] = 'Fix_(S | to)[a].
proof: Proof. by rewrite gacent1E setIA (setIidPl sSR). Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)