proposition: Lemma injm_pgroup pi A : A \subset D -> pi.-group (f @* A) = pi.-group A.
proof: Proof. by move=> sAD; rewrite /pgroup card_injm. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(fintype bigop finset prime fingroup morphism) mathcomp(gfunctor automorphism quotient action gproduct) mathcomp(cyclic)