code: func (rl *RateLimiter) Undo() {
  	current := atomic.AddUint64(&rl.allowance, rl.unit)
  
  	// Ensure our allowance is not over maximum
  	if max := atomic.LoadUint64(&rl.max); current > max {
  		atomic.AddUint64(&rl.allowance, max-current)
  	}
  }
docs: // Undo reverts the last Limit() call, returning consumed allowance