code: func (ls Set) String() string {
  	selector := make([]string, 0, len(ls))
  	for key, value := range ls {
  		selector = append(selector, key+"="+value)
  	}
  	// Sort for determinism.
  	sort.StringSlice(selector).Sort()
  	return strings.Join(selector, ",")
  }
docs: // String returns all fields listed as a human readable string.
  // Conveniently, exactly the format that ParseSelector takes.