query: SELECT
  ?item ?itemLabel 
  (GROUP_CONCAT(DISTINCT ?DatURL_language ; separator = " XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX ") as ?langOfReferenceSource)
  (GROUP_CONCAT(DISTINCT ?DatURL_reference_url ; separator = " XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX ") as ?DescAtURL_reference_url)
  (GROUP_CONCAT(DISTINCT ?DatURL_stated_in ; separator = " XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX ") as ?DescAtURL_stated_in)
  (GROUP_CONCAT(DISTINCT ?DatURL_retrieved ; separator = " XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX ") as ?DescAtURL_retrieved)
  WHERE {
    ?item p:P195 ?st .
    ?st  ps:P195 wd:Q1526131 .
    ?st pq:P2868 wd:Q29188408
    OPTIONAL {?item p:P973 [pq:P407 ?DatURL_language].} 
    OPTIONAL {?item p:P973 [prov:wasDerivedFrom [pr:P854 ?DatURL_reference_url]] .} 
    OPTIONAL {?item p:P973 [prov:wasDerivedFrom [pr:P248 ?DatURL_stated_in]] .} 
    OPTIONAL {?item p:P973 [prov:wasDerivedFrom [pr:P813 ?DatURL_retrieved]] .} 
    SERVICE wikibase:label { bd:serviceParam wikibase:language "nl". }
  } 
  GROUP BY ?item ?itemLabel 
  ORDER BY ?itemLabel