code: func (w *withStack) Format(s fmt.State, verb rune) {
  	switch verb {
  	case 'v':
  		if s.Flag('+') {
  			_, _ = fmt.Fprintf(s, "%+v", w.Cause())
  			return
  		}
  		fallthrough
  
  	case 's':
  		_, _ = io.WriteString(s, w.Error())
  
  	case 'q':
  		_, _ = fmt.Fprintf(s, "%q", w.Error())
  	}
  }
docs: // Format implements the fmt.Formatter interface.