Question: Who is the current leader of a nation where the United States Dollar is used as currency?
Answer(s):
answer: Raúl Castro

SPARQL query:
PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?x
WHERE {
FILTER (?x != ?c)
FILTER (!isLiteral(?x) OR lang(?x) = '' OR langMatches(lang(?x), 'en'))
?c ns:location.country.currency_formerly_used ns:m.09nqf . 
?c ns:government.governmental_jurisdiction.governing_officials ?y .
?y ns:government.government_position_held.office_holder ?x .
?y ns:government.government_position_held.basic_title ns:m.060c4 .
FILTER(NOT EXISTS {?y ns:government.government_position_held.from ?sk0} || 
EXISTS {?y ns:government.government_position_held.from ?sk1 . 
FILTER(xsd:datetime(?sk1) <= "2015-08-10"^^xsd:dateTime) })
FILTER(NOT EXISTS {?y ns:government.government_position_held.to ?sk2} || 
EXISTS {?y ns:government.government_position_held.to ?sk3 . 
FILTER(xsd:datetime(?sk3) >= "2015-08-10"^^xsd:dateTime) })
}