proposition: Lemma xsdprodm_dom1 : DgH \subset 'dom fsH.
proof: Proof. by rewrite ['dom _]morphpre_invm. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)