What are some party leaders of the parties which have had a seat in Berlin?
SELECT DISTINCT ?uri WHERE { ?x <http://dbpedia.org/ontology/headquarter> <http://dbpedia.org/resource/Berlin> . ?x <http://dbpedia.org/ontology/leader> ?uri  . ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/PoliticalParty>}
Berlin