proposition: Lemma nclasses_isog : G \isog H -> #|classes G| = #|classes H|.
proof: Proof. by case/isogP=> f injf <-; rewrite nclasses_injm. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup)