code: func countTags(n *Node) int {
  	count := 0
  	for _, e := range n.LabelTags {
  		if e.Flat != 0 {
  			count++
  		}
  	}
  	for _, t := range n.NumericTags {
  		for _, e := range t {
  			if e.Flat != 0 {
  				count++
  			}
  		}
  	}
  	return count
  }
docs: // countTags counts the tags with flat count. This underestimates the
  // number of tags being displayed, but in practice is close enough.