@property
    def _next_id(self):
        """
        Return the next available slide ID as an int. Valid slide IDs start
        at 256. Unused ids in the sequences starting from 256 are used first.
        """
        id_str_lst = self.xpath('./p:sldId/@id')
        used_ids = [int(id_str) for id_str in id_str_lst]
        for n in range(256, 258+len(used_ids)):
            if n not in used_ids:
                return n
dataset/ETHPy150Open scanny/python-pptx/pptx/oxml/parts/presentation.py/CT_SlideIdList._next_id/original