query: SELECT ?item WHERE {
  ?item wdt:P21 wd:Q6581072.  
  ?item wdt:P2454 ?knaw .
    OPTIONAL {
      ?wiki0 <http://schema.org/about> ?item .
      ?wiki0 <http://schema.org/isPartOf> <https://nl.wikipedia.org/> .
    }
    FILTER(!bound(?wiki0))
  }