Project(Filter(exists((join, (select, (bgp, (triple, <http://www.example.org/b>, ?y, <http://www.example.org/b>)), (?y)), (select, (bgp, (triple, ?x, <http://www.example.org/p>, <http://www.example.org/b>)), (?x)))) ,
               BGP( ?x <http://www.example.org/p> ?y)),
        {?x})