code: func NewCache(fs afero.Fs, maxAge time.Duration) *Cache {
  	return &Cache{
  		Fs:      fs,
  		nlocker: &lockTracker{Locker: locker.NewLocker(), seen: make(map[string]struct{})},
  		maxAge:  maxAge,
  	}
  }
docs: // NewCache creates a new file cache with the given filesystem and max age.