public function render(Renderer $renderer) {
    if (!$this->isDirectory() && is_null($this->getContent())) {
      $content = '';
      if ($header_template = $this->getHeaderTemplate()) {
        $content .= $renderer->render($header_template, $this->getVars()) . "\n";
      }
      $content .= $renderer->render($this->getTemplate(), $this->getVars());
      $this->content($content);
    }
  }