(select
 (leftjoin
  (bgp
    (triple <http://example/x1> <http://example/p> ?:v))
  (leftjoin
   (bgp
     (triple <http://example/x3> <http://example/q> ?:w))
   (bgp
     (triple <http://example/x2> <http://example/p> ?:v))))
 (?:v ?:w))