(select
 (leftjoin
  (bgp
    (triple <http://example.org/ns#p> <http://example.org/ns#q>
            <http://example.org/ns#r>))
  (bgp
    (triple <http://example.org/ns#a> <http://example.org/ns#b>
            <http://example.org/ns#c>)))
 COMMON-LISP:NIL)