proposition: Lemma card_orbit G x : #|orbit to G x| = #|G : 'C_G[x | to]|.
proof: Proof. by rewrite card_orbit_in ?subsetT. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)