(prefix ((: <http://example/>))
  (join
    (table (row (?o <http://example/z>)))
    (sequence
      (project (?s ?p)
      (bgp (triple ?s ?p <http://example/z>)))
    (bgp (triple ?s ?p <http://example/z>))
    ))

## 
##     (join
##       (table (row (?o <http://example/z>)))
##       (join
## 	(project (?s ?p)
## 	  (bgp (triple ?s ?p <http://example/z>))
## 	  )
## 	(bgp (triple ?s ?p ?o))))
##     )