code: func (f stringPairsFlag) Set(s string) error {
  	parts := strings.Split(s, ":")
  	if len(parts) == 1 {
  		return fmt.Errorf("no colon")
  	}
  	strpair.Map(f).Add(parts[0], parts[1])
  	return nil
  }
docs: // Set implements the flag.Value interface.