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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 set_file_idx_offset(self, file_idx_offset=0):
 """Set offset of file index. Args: file_idx_offset: It can be either an integer or 'auto'. If set to an integer, the filename will start from ``file_idx_offset`` + 1. If set to ``'auto'``, the filename will start from existing max file index plus 1. """

        if isinstance(file_idx_offset, int):
            self.file_idx_offset = file_idx_offset
        elif file_idx_offset == 'auto':
            self.file_idx_offset = self.storage.max_file_idx()
        else:
            raise ValueError('"file_idx_offset" must be an integer or `auto`')