proposition: Lemma im_dprodm : dprodm @* G = fH @* H * fK @* K.
proof: Proof. exact: im_cprodm. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)