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