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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 diff(self, summary1=None, summary2=None):
 """Compute diff between to summaries. If no summary is provided, the diff from the last to the current summary is used. If summary1 is provided the diff from summary1 to the current summary is used. If summary1 and summary2 are provided, the diff between these two is used. """

        res = None
        if summary2 is None:
            self.s1 = self.create_summary()
            if summary1 is None:
                res = summary.get_diff(self.s0, self.s1)
            else:
                res = summary.get_diff(summary1, self.s1)
            self.s0 = self.s1
        else:
            if summary1 is not None:
                res = summary.get_diff(summary1, summary2)
            else:
                raise ValueError("You cannot provide summary2 without summary1.")
        return summary._sweep(res)