code: func NewMergeSeriesSet(sets []SeriesSet, querier *mergeQuerier) SeriesSet {
  	if len(sets) == 1 {
  		return sets[0]
  	}
  
  	// Sets need to be pre-advanced, so we can introspect the label of the
  	// series under the cursor.
  	var h seriesSetHeap
  	for _, set := range sets {
  		if set == nil {
  			continue
  		}
  		if set.Next() {
  			heap.Push(&h, set)
  		}
  	}
  	return &mergeSeriesSet{
  		heap:    h,
  		sets:    sets,
  		querier: querier,
  	}
  }
docs: // NewMergeSeriesSet returns a new series set that merges (deduplicates)
  // series returned by the input series sets when iterating.