@Override
 protected PrefixMapping view() {
   PrefixMapStorage view = PrefixMapStorageView.viewGraph(dsgprefixes, gn);
   PrefixMapI pmap = PrefixesFactory.newPrefixMap(view);
   return PrefixesFactory.newPrefixMappingOverPrefixMapI(pmap);
 }
 @Override
 protected PrefixMapping create() {
   dsgprefixes = PrefixesFactory.newDatasetPrefixesMem();
   return view();
 }