Question: Who was the wife of U.S. president Lincoln?
SPARQL query:
PREFIX onto: <http://dbpedia.org/ontology/>
PREFIX res: <http://dbpedia.org/resource/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT DISTINCT ?uri ?string 
WHERE {
	res:Abraham_Lincoln onto:spouse ?uri.
        OPTIONAL { ?uri rdfs:label ?string . FILTER (lang(?string) = 'en') }
}