What is {software engine} of {Dota 2} that is {end time} is {2015-9-0} ?
What engine was Dota 2 based on at the start of September 2015?
SELECT ?obj WHERE { wd:Q771541 p:P408 ?s . ?s ps:P408 ?obj . ?s pq:P582 ?x filter(contains(YEAR(?x),'2015')) }
select distinct ?obj  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q771541> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P408> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj .
?statement <http://www.wikidata.org/entity/P582> <2015-9-0>
} 

Give the name of the engine  the Dota 2 was based on as of September 2015.