code: def diff(self, summary1=None, summary2=None):
          
          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)
docs: 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.