proposition: Lemma sdprodm_sub : G \subset H <*> K.
proof: Proof. by case/sdprodP: eqHK_G => _ <- nHK _; rewrite norm_joinEr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)