划出UML类图：一本教材由许多章组成，每一章由许多节、小结和习题组成，章和节都有标题和序号属性。

下面是一本教材的UML类图：

+------------------+             +-------------+
|    Textbook      |             |    Chapter  |
+------------------+             +-------------+
| -title: string   |             | -title: string
| -author: string  |             | -number: int
| -chapters: array |             | -sections: array
+------------------+             +-------------+
| +setTitle(title: string): void | +setTitle(title: string): void
| +setAuthor(author: string): void| +setNumber(number: int): void
| +addChapter(chapter: Chapter): void | +addSection(section: Section): void
| +getChapters(): array          | +getSections(): array
+------------------+             +-------------+
         |                               |
         | has a                         | has a
         |                               |
+------------------+             +-------------+
|     Chapter      |             |    Section  |
+------------------+             +-------------+
| -title: string   |             | -title: string
| -number: int     |             | -number: int
| -sections: array |             | -exercises: array
+------------------+             +-------------+
| +setTitle(title: string): void | +setTitle(title: string): void
| +setNumber(number: int): void  | +setNumber(number: int): void
| +addSection(section: Section): void | +addExercise(exercise: Exercise): void
| +getSections(): array          | +getExercises(): array
+------------------+             +-------------+
         |                               |
         | has a                         | has a
         |                               |
+------------------+             +-------------+
|     Section      |             |   Exercise  |
+------------------+             +-------------+
| -title: string   |             | -question: string
| -number: int     |             | -answer: string
| -exercises: array|             +-------------+
+------------------+             | +setQuestion(question: string): void
| +setTitle(title: string): void | +setAnswer(answer: string): void
| +setNumber(number: int): void  | +getQuestion(): string
| +addExercise(exercise: Exercise): void | +getAnswer(): string
| +getExercises(): array         |
+------------------+             |
                                 |
                                 |
                          +-------------+
                          |   Bookstore |
                          +-------------+
                          | -books: array|
                          +-------------+
                          | +addBook(book: Textbook): void
                          | +getBooks(): array
                          +-------------+

其中，`Textbook`类表示教材，包含属性`title`、`author`和`chapters`，以及方法`setTitle`、`setAuthor`、`addChapter`和`getChapters`。`Chapter`类表示章节，包含属性`title`、`number`和`sections`，以及方法`setTitle`、`setNumber`、`addSection`和`getSections`。`Section`类表示小节，包含属性`title`、`number`和`exercises`，以及方法`setTitle`、`setNumber`、`addExercise`和`getExercises`。`Exercise`类表示习题，包含属性`question`和`answer`，以及方法`setQuestion`、`setAnswer`、`getQuestion`和`getAnswer`。`Bookstore`类表示书店，包含属性`books`，以及方法`addBook`和`getBooks`。其中，`books`是`Textbook`对象的数组。