What is the {doctoral thesis} for {doctoral thesis} of {Helmut Kohl}
What was the subject of Hemut Kohl's doctoral thesis?
 select distinct ?obj where { wd:Q2518 wdt:P1026 ?obj . ?obj wdt:P31 wd:Q187685 } 
select distinct ?obj where { ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q2518> . ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P1026> . ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj . ?obj <http://www.wikidata.org/entity/P31> <http://wikidata.dbpedia.org/resource/Q187685> } 
Give me the subject of Hemut Kohl's doctoral thesis.