Question: Give me all libraries established earlier than 1400.
SPARQL query:
PREFIX onto: <http://dbpedia.org/ontology/>
PREFIX prop: <http://dbpedia.org/property/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT DISTINCT ?uri
WHERE {
        ?uri rdf:type onto:Library .
        ?uri prop:established ?year . 
        FILTER (?year < 1400)
}