code: func lessonHandler(w http.ResponseWriter, r *http.Request) {
  	lesson := strings.TrimPrefix(r.URL.Path, "/lesson/")
  	if err := writeLesson(lesson, w); err != nil {
  		if err == lessonNotFound {
  			http.NotFound(w, r)
  		} else {
  			log.Println(err)
  		}
  	}
  }
docs: // lessonHandler handler the HTTP requests for lessons.