code: func NewRateLimitedStorage(storage Storage, maxConcurrency int) Storage {
  	if maxConcurrency < 1 {
  		return storage
  	}
  
  	return &RateLimitedStorage{
  		semaphore: make(chan bool, maxConcurrency),
  		storage:   storage,
  	}
  }
docs: // This returns a RateLimitedStorage.  This is useful for cases where
  // high concurrent load may degrade the underlying storage's performance.
  // NOTE: when maxConcurrency is non-positive, the original storage is returned
  // (i.e., the storage is not rate limited).