Question: Who was the president at the time when the opponent of Benjamin Spock held office?
SPARQL query:
PREFIX res: <http://dbpedia.org/resource/>
PREFIX prop: <http://dbpedia.org/property/>
PREFIX onto: <http://dbpedia.org/ontology/>
SELECT DISTINCT ?uri WHERE { ?x onto:opponent res:Benjamin_Spock . ?x prop:incumbent ?uri  . }