code: def open_tunnel(self, serial_no, port=19020):
          
          return self.open(ip_addr='tunnel:' + str(serial_no) + ':' + str(port))
docs: Connects to the J-Link emulator (over SEGGER tunnel).
  
          Args:
            self (JLink): the ``JLink`` instance
            serial_no (int): serial number of the J-Link
            port (int): optional port number (default to 19020).
  
          Returns:
            ``None``