query: SELECT DISTINCT ?item ?head ?office ?officeholder ?person 
  WHERE {      
    ?item wdt:P31 wd:Q3624078 ; wdt:P1313 ?office .
    FILTER NOT EXISTS { ?office wdt:P576 ?end_date } .
    FILTER (?item != wd:Q238) 
  
    OPTIONAL {
      ?person p:P39 ?posn ; wdt:P31 wd:Q5 .
      ?posn ps:P39 ?office ; pq:P580 ?start_date .
      FILTER (?start_date < NOW()) .
      OPTIONAL { ?posn pq:P582 ?end_date }  
      FILTER ( !bound(?end_date)