code: def summary(self, prn=None, lfilter=None):
          
          for s, r in self.res:
              if lfilter is not None:
                  if not lfilter(s, r):
                      continue
              if prn is None:
                  print(self._elt2sum((s, r)))
              else:
                  print(prn(s, r))
docs: prints a summary of each SndRcv packet pair
  prn:     function to apply to each packet pair instead of lambda s, r: "%s ==> %s" % (s.summary(),r.summary())
  lfilter: truth function to apply to each packet pair to decide whether it will be displayed