query: SELECT DISTINCT ?item ?office ?person ?replaces
  WHERE { 
    ?item wdt:P31 wd:Q3624078 ; wdt:P1313 ?office . 
    FILTER NOT EXISTS { ?office wdt:P576 ?end_date } .
    OPTIONAL {
      ?person p:P39 ?posn ; wdt:P31 wd:Q5 .
      ?posn ps:P39 ?office ; pq:P580 ?start_date .
      OPTIONAL { ?posn pq:P582 ?end_date }  
      FILTER ( !bound(?end_date)