code: func NATManager(nm config.NATManagerC) Option {
  	return func(cfg *Config) error {
  		if cfg.NATManager != nil {
  			return fmt.Errorf("cannot specify multiple NATManagers")
  		}
  		cfg.NATManager = nm
  		return nil
  	}
  }
docs: // NATManager will configure libp2p to use the requested NATManager. This
  // function should be passed a NATManager *constructor* that takes a libp2p Network.