code: func MyIp4() *net.IPAddr {
  	myIp4Once.Do(func() {
  		var err error
  		myIp4, err = net.ResolveIPAddr("ip4", MyHostname())
  		if err != nil {
  			log.Fatal(err)
  		}
  	})
  	return myIp4
  }
docs: // Resolves `MyHostname()` to an Ip4 address. Caches first successful result, making it
  // cheap to call it over and over.
  // It will also crash whole process if resolving the IP fails!