code: func (fsc *FSCache) RegisterTransport(id string, transport Transport) error {
  	fsc.mu.Lock()
  	defer fsc.mu.Unlock()
  	if _, ok := fsc.transports[id]; ok {
  		return errors.Errorf("transport %v already exists", id)
  	}
  	fsc.transports[id] = transport
  	return nil
  }
docs: // RegisterTransport registers a new transport method