code: func NewFSCache(opt Opt) (*FSCache, error) {
  	store, err := newFSCacheStore(opt)
  	if err != nil {
  		return nil, err
  	}
  	return &FSCache{
  		store:      store,
  		opt:        opt,
  		transports: make(map[string]Transport),
  	}, nil
  }
docs: // NewFSCache returns new FSCache object