def nsummary(self,prn=None, lfilter=None):
        """prints a summary of each packet with the packet's number
prn:     function to apply to each packet instead of lambda x:x.summary()
lfilter: truth function to apply to each packet to decide whether it will be displayed"""
        for i, p in enumerate(self.res):
            if lfilter is not None:
                if not lfilter(p):
                    continue
            print(conf.color_theme.id(i,fmt="%04i"), end = " ")
            if prn is None:
                print(self._elt2sum(p))
            else:
                print(prn(p))