add minus sub exhibitions

services/exhibitions/queries/main.query

@@ -12,6 +12,8 @@ SELECT DISTINCT ?uri ?label ?description ?logo ?alternative WHERE {
     OPTIONAL { ?uri dct:alternative ?alternative . }
     OPTIONAL { ?uri dct:description ?description . }
     OPTIONAL { ?uri foaf:logo ?logo . }
+ 
+    MINUS { ?uri dct:isPartOf ?partOf . }
      
   
 # uncomment the next line, and insert uri for exhibition to skip