Add an internal value to a string

def addfield(self, pkt, s, val):
        """Add an internal value to a string"""
        if self.adjust(pkt, self.length_of) == 2:
            return s + struct.pack(self.fmt[0] + "H", val)
        elif self.adjust(pkt, self.length_of) == 8:
            return s + struct.pack(self.fmt[0] + "Q", val)
        else:
            return s