code: func (e *merryErr) Format(s fmt.State, verb rune) {
  	switch verb {
  	case 'v':
  		if s.Flag('+') {
  			io.WriteString(s, Details(e))
  			return
  		}
  		fallthrough
  	case 's':
  		io.WriteString(s, e.Error())
  	case 'q':
  		fmt.Fprintf(s, "%q", e.Error())
  	}
  }
docs: // Format implements fmt.Formatter