(select
 (join
  (bgp
    (triple <http://example.org/x> <http://example.org/p>
            <http://example.org/z>))
  (graph ?::g
         (join
          (bgp
            (triple <http://example.org/x> <http://example.org/b> ?:a))
          (graph ?::g2
                 (bgp
                   (triple <http://example.org/x> <http://example.org/p>
                           ?:x))))))
 (?::g ?:a ?::g2 ?:x))