code: func (f Fields) String() string {
  	b := bytes.Buffer{}
  	b.WriteRune('{')
  	for idx, e := range f.SortedEntries() {
  		if idx > 0 {
  			b.WriteString(", ")
  		}
  		b.WriteString(e.String())
  	}
  	b.WriteRune('}')
  	return b.String()
  }
docs: // String returns a string describing the contents of f in a sorted,
  // dictionary-like format.