code: func (pl *PairList) String() string {
  	ps := make([]string, 0, len(pl.Pairs))
  	for _, k := range pl.Keys() {
  		ps = append(ps, fmt.Sprintf("%v=%v", k, pl.Pairs[k]))
  	}
  	return strings.Join(ps, " ")
  }
docs: // String presents a k=v list in key=sorted order