Project(Filter(exists((join, (select, (table, unit), ((?y, <http://www.example.org/e>))), (select, (table, unit), ((?x, <http://www.example.org/d>))))) ,
               Extend(Extend({ }, ?x, <http://www.example.org/b>),
                      ?y,
                      <http://www.example.org/b>)),
        {?x})