query: SELECT ?item (COUNT(?omvat) AS ?omvatCount) {
    ?item wdt:P6379 ?collectie.
    ?item wdt:P31/(wdt:P279)* wd:Q10648343.
    OPTIONAL {
     ?item wdt:P527 ?omvat.
    }
  }
  GROUP BY ?item
  HAVING (?omvatCount < 2)