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    // $id = chapter_id 

    try {
        $chapter = $DB->get_record('book_chapters', array('id'=>$id), '*', MUST_EXIST);
        $book = $DB->get_record('book', array('id'=>$chapter->bookid), '*', MUST_EXIST);
        $cm = get_coursemodule_from_instance('book', $book->id, $book->course, MUST_EXIST);
        $course = $DB->get_record('course', array('id'=>$cm->course), '*', MUST_EXIST);
    } catch (dml_missing_record_exception $ex) {
        return NULL;
    }

    try {
        $context = context_module::instance($cm->id);
        if (!$cm->visible && !has_capability('mod/book:viewhiddenchapters', $context)) {
            return NULL;
        }
        require_capability('mod/book:read', $context);
    } catch (moodle_exception $ex) {
        return NULL;
    }
    return TRUE;