Question: Which pro athletes started their careers before 2013 and are in the current Boston Celtics' roster?
Answer(s):
answer: Jeff Green
answer: Gerald Wallace
answer: Brandon Bass
answer: Avery Bradley

SPARQL query:
PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?x
WHERE {
FILTER (?x != ns:m.0bwjj)
FILTER (!isLiteral(?x) OR lang(?x) = '' OR langMatches(lang(?x), 'en'))
ns:m.0bwjj ns:sports.sports_team.roster ?y .
?y ns:sports.sports_team_roster.player ?x .
FILTER(NOT EXISTS {?y ns:sports.sports_team_roster.from ?sk0} || 
EXISTS {?y ns:sports.sports_team_roster.from ?sk1 . 
FILTER(xsd:datetime(?sk1) <= "2015-08-10"^^xsd:dateTime) })
FILTER(NOT EXISTS {?y ns:sports.sports_team_roster.to ?sk2} || 
EXISTS {?y ns:sports.sports_team_roster.to ?sk3 . 
FILTER(xsd:datetime(?sk3) >= "2015-08-10"^^xsd:dateTime) })
?x ns:sports.pro_athlete.career_start ?num .
FILTER (?num < "2013"^^xsd:dateTime) . 
}