PREFIX  jfn:  <java:com.hp.hpl.jena.query.function.>
PREFIX  jx:   <java:com.hp.hpl.jena.query.extension.>
PREFIX  xsd:  <http://www.w3.org/2001/XMLSchema#>

SELECT *
WHERE
  { ?x ?y ?v . 
   # FILTER jfn:Debug(?v+1)
   EXT jx:Debug(?v+1)
  }