code: func funcAbsent(vals []Value, args Expressions, enh *EvalNodeHelper) Vector {
  	if len(vals[0].(Vector)) > 0 {
  		return enh.out
  	}
  	m := []labels.Label{}
  
  	if vs, ok := args[0].(*VectorSelector); ok {
  		for _, ma := range vs.LabelMatchers {
  			if ma.Type == labels.MatchEqual && ma.Name != labels.MetricName {
  				m = append(m, labels.Label{Name: ma.Name, Value: ma.Value})
  			}
  		}
  	}
  	return append(enh.out,
  		Sample{
  			Metric: labels.New(m...),
  			Point:  Point{V: 1},
  		})
  }
docs: // === absent(Vector ValueTypeVector) Vector ===