PREFIX  dc:   <http://purl.org/dc/elements/1.1/>
PREFIX  cal:  <http://www.example.org/Calendar#>
PREFIX  rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX : <.>

SELECT ?title1 ?start1 ?sTime1 ?end1 ?eTime1 ?location
FROM NAMED <SRobert.rdf>
FROM NAMED <STamara.rdf>

WHERE {

  GRAPH :SRobert.rdf {
    ?x dc:title ?title1.
    ?x cal:date ?y.
    ?y cal:startDay ?start1.
    ?y cal:startTime ?sTime1.
    ?y cal:endTime ?eTime1.
    ?y cal:endDay ?end1.
    optional { ?x cal:location ?location. }
  
    FILTER (!bound(?location))
  }.
    
}