proposition: Lemma afix1P a x : reflect (to x a = x) (x \in 'Fix_to[a]).
proof: Proof. by rewrite inE sub1set inE; apply: eqP. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)