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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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