proposition: Lemma afixU A B : 'Fix_to(A :|: B) = 'Fix_to(A) :&: 'Fix_to(B).
proof: Proof. by apply/setP=> x; rewrite !inE subUset. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)