proposition: Lemma sub_isog (A : {set aT}) : A \subset G -> 'injm f -> isog A (f @* A).
proof: Proof. by move=> sAG injf; apply: (isom_isog f sAG); apply: sub_isom. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup)