PREFIX : <http://example/>
SELECT *
{
# urn:x-arq:DefaultGraph
# urn:x-arq:UnionGraph

  { ?s ?p ?o . LET (?g := "Union") } 
 UNION 
   { GRAPH <urn:x-arq:DefaultGraph> { ?s ?p ?o LET (?g := "Explicit") } }



}