code: func (idx WaitIndexVal) Next(previous BlockingParamVal) BlockingParamVal {
  	if previous == nil {
  		return idx
  	}
  	prevIdx, ok := previous.(WaitIndexVal)
  	if ok && prevIdx > idx {
  		// This value is smaller than the previous index, reset.
  		return WaitIndexVal(0)
  	}
  	return idx
  }
docs: // Next implements BlockingParamVal