code: def add_msg(self, pkt):
          
          if not self.buffer_out:
              self.add_record()
          r = self.buffer_out[-1]
          if isinstance(r, TLS13):
              self.buffer_out[-1].inner.msg.append(pkt)
          else:
              self.buffer_out[-1].msg.append(pkt)
docs: Add a TLS message (e.g. TLSClientHello or TLSApplicationData)
          inside the latest record to be sent through the socket.
          We believe a good automaton should not use the first test.