Question: How many non-fiction subject are there of the books whose one of the non-fiction subject is Ankh-Morpork City Watch ?
SPARQL:
 SELECT DISTINCT COUNT(?uri) WHERE { ?x <http://dbpedia.org/ontology/nonFictionSubject> <http://dbpedia.org/resource/Ankh-Morpork_City_Watch> . ?x <http://dbpedia.org/ontology/nonFictionSubject> ?uri }