public function getNavHTML($content_label){
		if( !$this->root ){
			return '';
		}
		$landmarks = '';
		$toc = '';
		foreach( $this->children as $child ){
			$toc .= $this->makeList($child, false);
			$landmarks .= $this->makeList($child, true, $content_label);
		}
		$html = <<<HTML
<nav epub:type="toc" id="toc">
	<ol>
		{$toc}
	</ol>
</nav>

<nav id="landmarks" epub:type="landmarks" hidden="hidden" class="hidden">
	<ol epub:type="list">
		{$landmarks}
	</ol>
</nav>
HTML;
		return trim($html);
	}