code: func (t TagMap) SortedKeys() []string {
  	if len(t) == 0 {
  		return nil
  	}
  
  	keys := make([]string, 0, len(t))
  	for k := range t {
  		keys = append(keys, k)
  	}
  	sort.Strings(keys)
  	return keys
  }
docs: // SortedKeys returns a sorted slice of the keys in a TagMap.