code: def _next_id(self):
          
          id_str_lst = self.xpath('./p:sldId/@id')
          return max([255]+[int(id_str) for id_str in id_str_lst])+1
docs: Return the next available slide ID as an int. Valid slide IDs start
          at 256. The next integer value greater than the max value in use is
          chosen, which minimizes that chance of reusing the id of a deleted
          slide.