proposition: Lemma HallP G H : Hall G H -> exists pi, pi.-Hall(G) H.
proof: Proof. by exists \pi(H); apply: Hall_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)