docs: 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