Question: What does the river flow into, whose left tributary is Bukovica?
SPARQL query: SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/ontology/leftTributary> <http://dbpedia.org/resource/Bukovica_(river)> . ?x <http://dbpedia.org/ontology/mouthCountry> ?uri  . }
Answer: [Croatia] is the mouth country of the river whose left tributary is Bukovica.