You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: igd_service.go
    path of file: ./repos/syncthing/lib/upnp
    
    the code of the file until where you have to start completion: // Copyright (C) 2016 The Syncthing Authors.
//
// Adapted from https://github.com/jackpal/Taipei-Torrent/blob/dd88a8bfac6431c01d959ce3c745e74b8a911793/IGD.go
// Copyright (c) 2010 Jack Palevich (https://github.com/jackpal/Taipei-Torrent/blob/dd88a8bfac6431c01d959ce3c745e74b8a911793/LICENSE)
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditio
    
func (s *IGDService) GetExternalIPv4Address(ctx context.Context) (net.IP, error) {
	const template = `<u:GetExternalIPAddress xmlns:u="%s" />`

	body := fmt.Sprintf(template, s.URN)
	response, err := soapRequest(ctx, s.URL, s.URN, "GetExternalIPAddress", body)
	if err != nil {
		return nil, err
	}

	var envelope soapGetExternalIPAddressResponseEnvelope
	if err := xml.Unmarshal(response, &envelope); err != nil {
		return nil, err
	}

	result := net.ParseIP(envelope.Body.GetExternalIPAddressResponse.NewExternalIPAddress)

	return result, nil
}