proposition: Lemma pcore_sub_Hall G H : pi.-Hall(G) H -> 'O_pi(G) \subset H.
proof: Proof. by move/Hall_max=> maxH; apply: bigcap_inf. 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)