PREFIX ical: <http://www.w3.org/2002/12/cal/icaltzd#>
PREFIX xs: <http://www.w3.org/2001/XMLSchema#>
SELECT ?start1 ?stop1 ?loc1 ?summ1 ?summ2 

FROM <http://www.w3.org/2001/sw/grddl-wg/doc29/david-erdf.rdf>
FROM <http://www.w3.org/2001/sw/grddl-wg/doc29/robin-hcal-grddl.rdf>

WHERE
  { 
	?event1 a ical:Vevent;
              ical:summary ?summ1 ;
  	      ical:dtstart ?start1 ;
              ical:dtend ?stop1 ;
              ical:location ?loc1.
  	?event2 a ical:Vevent;
              ical:summary ?summ2 ;
  	      ical:dtstart ?start2;
              ical:dtend ?stop2;
              ical:location ?loc2.
	FILTER (  ?event1 != ?event2) .
      FILTER ( xs:string(?start1) = xs:string(?start2) ).
      FILTER ( xs:string(?stop1) = xs:string(?stop2) ).
      FILTER ( xs:string(?loc1) = xs:string(?loc2) ).
}