proposition: Lemma Hall_pi G H : Hall G H -> \pi(H).-Hall(G) H.
proof: Proof. by case/andP=> sHG coHG /=; rewrite /pHall sHG /pgroup pnat_pi -?coprime_pi'. 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)