code: func stringSetSlice(m map[string]struct{}) []string {
  	if m == nil {
  		return nil
  	}
  
  	a := make([]string, 0, len(m))
  	for k := range m {
  		a = append(a, k)
  	}
  	sort.Strings(a)
  	return a
  }
docs: // stringSetSlice returns a sorted slice of keys from a string set.