Why lazy-loading getIterator does not work

gistfile1.aw

// Our book class. We can still iterate over our chapters with the
// help of the iteratorAggregate implementation.
class Book implements IteratorAggregate {
    protected $_title;
    protected $_author;
    protected $_chapters;

    protected $_it = null;

    function __construct($title, $author) {
        $this->_title = $title;
        $this->_author = $author;
        $this->_chapter = array();
    }

    // This method must be implemented. It will return the actual iterator
    function getIterator()
    {
        // Lazy loading the iterator
        if (! $this->_it) {
            // This will return the articles. Since they are inside an array, we
            // can use the standard array-iterator.
            $this->_it =  new ArrayIterator($this->_chapters);
        }
        return $this->_it;
    }

    // Add a new chapter to this book
    function addChapter(Chapter $chapter) {
        $this->_chapters[] = $chapter;
    }

    function getTitle() {
        return $this->_title;
    }

    function getAuthor() {
        return $this->_author;
    }
}

// Create a new book and add chapters to it
$book = new Book("About the iteratorAggregate", "Joshua Thijssen");
$book->addChapter(new Chapter("Foreword", "This is the introduction"));
$book->addChapter(new Chapter("Chapter 1", "Content"));
$book->addChapter(new Chapter("Chapter 2", "Content"));

print "All chapters from the book '" . $book->getTitle() . "', written by ".$book->getAuthor()." :" . PHP_EOL;
foreach ($book as $chapter) {
    print "- " . $chapter->getTitle() . PHP_EOL;
}

// Add another chapter to our book
$book->addChapter(new Chapter("Epilogue", "Famous last words"));

// We cannot see the new chapter in the next loop, since our Book uses the already-initialized iterator.
print "All chapters from the book '" . $book->getTitle() . "', written by ".$book->getAuthor()." :" . PHP_EOL;
foreach ($book as $chapter) {
    print "- " . $chapter->getTitle() . PHP_EOL;
}