code: func (r *RateLimiter) Left(id string) (int64, error) {
  	// Retrieve attempted count.
  	attempted, err := r.Attempted(id)
  	if err != nil {
  		return 0, err
  	}
  
  	// Left is max minus attempted.
  	left := r.max - attempted
  	if left < 0 {
  		return 0, nil
  	}
  
  	return left, nil
  }
docs: // Left returns the number of remaining requests for id during a current
  // period.