code: func FilterUndeletedSeriesIDIterator(sfile *SeriesFile, itr SeriesIDIterator) SeriesIDIterator {
  	if itr == nil {
  		return nil
  	}
  	return &filterUndeletedSeriesIDIterator{sfile: sfile, itr: itr}
  }
docs: // FilterUndeletedSeriesIDIterator returns an iterator which filters all deleted series.