func main() {
	line := &lineMessenger{
		channelID:     "<line-channel-id>",
		channelSecret: "<line-channel-secret>",
	}

	if !line.accessTokenExists() {
		err := line.issueAccessToken()
		if err != nil {
			log.Fatal(err)
		}
	}

	err := line.sendTextMessage("<line-user-id>", textMessages{
		newTextMessage("Hello, world!"),
	})
	if err != nil {
		log.Fatal(err)
	}
}