code: func PMP(gateway net.IP) Interface {
  	if gateway != nil {
  		return &pmp{gw: gateway, c: natpmp.NewClient(gateway)}
  	}
  	return startautodisc("NAT-PMP", discoverPMP)
  }
docs: // PMP returns a port mapper that uses NAT-PMP. The provided gateway
  // address should be the IP of your router. If the given gateway
  // address is nil, PMP will attempt to auto-discover the router.