proposition: Lemma sum_card_class : \sum_(C in classes G) #|C| = #|G|.
proof: Proof. by apply: acts_sum_card_orbit; apply/actsP=> x Gx y; apply: groupJr. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(fintype bigop finset fingroup morphism perm) mathcomp(automorphism quotient)