code: func ListenAddress() (string, error) {
  	listenAddr := os.Getenv("CAMLI_APP_LISTEN")
  	if listenAddr == "" {
  		return "", errors.New("CAMLI_APP_LISTEN is undefined")
  	}
  	return listenAddr, nil
  }
docs: // ListenAddress returns the host:[port] network address, derived from the environment,
  // that the application should listen on.