proposition: Lemma im_sdprodm1 : gH @* DgH = fH @* H.
proof: Proof. by rewrite morphim_restrm setIid morphim_comp im_invm. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)