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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 finish(self):
 """Finalize the MAR file. The MAR header, index and signatures need to be updated once we've finished adding all the files. """

        # Update the last_offset in the mar header
        self.write_header()
        # Write out the index of contents
        self.write_index()

        if not self.use_old_format:
            # Refresh the signature
            sigs = self.calculate_signatures()
            self.write_signatures(sigs)