code: func writeLesson(name string, w io.Writer) error {
  	if uiContent == nil {
  		panic("writeLesson called before successful initTour")
  	}
  	if len(name) == 0 {
  		return writeAllLessons(w)
  	}
  	l, ok := lessons[name]
  	if !ok {
  		return lessonNotFound
  	}
  	_, err := w.Write(l)
  	return err
  }
docs: // writeLesson writes the tour content to the provided Writer.