code: def ListClientsForKeywords(self, keywords, start_time=None):
      
      res = {kw: [] for kw in keywords}
      for kw in keywords:
        for client_id, timestamp in iteritems(self.keywords.get(kw, {})):
          if start_time is not None and timestamp < start_time:
            continue
          res[kw].append(client_id)
      return res
docs: Lists the clients associated with keywords.