code: func valueOrEnv(config map[string]string, key, env string) string {
  	if v := config[key]; v != "" {
  		return v
  	}
  	if v := os.Getenv(env); v != "" {
  		logger.Printf("[DEBUG] Using value of %s for configuration of %s", env, key)
  		return v
  	}
  	return ""
  }
docs: // valueOrEnv provides a way of suppling configuration values through
  // environment variables. Defined values always take priority.