proposition: Lemma pgroupS pi G H : H \subset G -> pi.-group G -> pi.-group H.
proof: Proof. by move=> sHG; apply: pnat_dvd (cardSg sHG). 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)