proposition: Lemma norm_joinEr G H : H \subset 'N(G) -> G <*> H = G * H.
proof: Proof. by move/normC=> cHG; apply: comm_joingE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)