code: func inversePostingsForMatcher(ix IndexReader, m labels.Matcher) (index.Postings, error) {
  	tpls, err := ix.LabelValues(m.Name())
  	if err != nil {
  		return nil, err
  	}
  
  	var res []string
  	for i := 0; i < tpls.Len(); i++ {
  		vals, err := tpls.At(i)
  		if err != nil {
  			return nil, err
  		}
  
  		if !m.Matches(vals[0]) {
  			res = append(res, vals[0])
  		}
  	}
  
  	var rit []index.Postings
  	for _, v := range res {
  		it, err := ix.Postings(m.Name(), v)
  		if err != nil {
  			return nil, err
  		}
  
  		rit = append(rit, it)
  	}
  
  	return index.Merge(rit...), nil
  }
docs: // inversePostingsForMatcher eeturns the postings for the series with the label name set but not matching the matcher.