proposition: Lemma cprodE G H : H \subset 'C(G) -> G \* H = G * H.
proof: Proof. by move=> cGH; rewrite /cprod cGH pprodE ?cents_norm. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)