code: func (l *List) String() string {
  	var o []string
  	for _, key := range l.sort {
  		cap := l.m[Capability(key)]
  		if len(cap.Values) == 0 {
  			o = append(o, key)
  			continue
  		}
  
  		for _, value := range cap.Values {
  			o = append(o, fmt.Sprintf("%s=%s", key, value))
  		}
  	}
  
  	return strings.Join(o, " ")
  }
docs: // String generates the capabilities strings, the capabilities are sorted in
  // insertion order