ite /= mulgA mulG_subG centM subsetI cGH cHK andbT -(cent_joinEr cHK).
by rewrite -(cent_joinEr cGH) !groupP.
Qed.

(* Specifications: *)
rewrite mem_enum; apply/idP/imag
Lemma dprodYP K H : reflect (K \x
Lemma dprodC : commutative dprod.