Question: Who was the wife of President Lincoln?
SPARQL query:
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX onto: <http://dbpedia.org/ontology/>
SELECT ?uri ?string
WHERE 
{
	?person rdf:type onto:President .
	?person foaf:surname 'Lincoln'@en .
	?person onto:spouse ?uri.
        OPTIONAL {?uri rdfs:label ?string . FILTER (lang(?string) = 'en') }
}