(select
 (filter
  (leftjoin
   (bgp
     (triple ?:book <http://purl.org/dc/elements/1.1/title> ?:title))
   (bgp
     (triple ?:book <http://example.org/ns#price> ?:price)))
  (|\|\|| (! (bound ?:price)) (:< ?:price 15)))
 (?:title ?:price))