(select
 (join
  (leftjoin
   (bgp
     (triple ?:a <http://example.org/ns#b> <http://example.org/ns#c>))
   (bgp
     (triple <http://example.org/ns#x> <http://example.org/ns#y>
             <http://example.org/ns#z>)))
  (bgp
    (triple <http://example.org/ns#x> ?:y ?:z)))
 (?:a ?:y ?:z))