def _next_id(self):
        """
        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.
        """
        id_str_lst = self.xpath('./p:sldId/@id')
        return max([255]+[int(id_str) for id_str in id_str_lst])+1
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.