public function addPage(HTML_QuickForm2_Controller_Page $page)
    {
        $pageId = $page->getForm()->getId();
        if (!empty($this->pages[$pageId])) {
            throw new HTML_QuickForm2_InvalidArgumentException(
                "Duplicate page ID '{$pageId}'"
            );
        }
        $page->setController($this);
        $this->pages[$pageId] = $page;
    }