(select
 (filter
  (bgp
    (triple ?:x1 <http://example.org/things#p> ?:v1)
    (triple ?:x2 <http://example.org/things#p> ?:v2))
  (sameTerm ?:v1 ?:v2))
 (?:x1 ?:v1 ?:x2 ?:v2))