(select
 (filter
  (bgp
    (triple ?:a <http://example.org/ns#p> ?:v))
  (&&
   #S(SPOCQ:BOOLEAN
      :ID COMMON-LISP:NIL
      :LEXICAL-FORM "true"
      :VALUE COMMON-LISP:T)
   ?:v))
 (?:a))