Distinct(Project(Join(Graph(<http://schema.org/version/2.1/>,
                            BGP( ?b <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Class>)),
                      Graph(?xg,
                            BGP( ?a <http://www.w3.org/2000/01/rdf-schema#subClassOf> ?b .
                                 ?a <http://schema.org/isPartOf> ?ext))),
                 {?a, ?b, ?ext, ?xg}))