/**
  * Return the opening markup for multiple navigation items
  *
  * @return  string
  */
 public function beginChildrenMarkup()
 {
     $cssClass = array(static::CSS_CLASS_NAV);
     if ($this->navigation->getLayout() === Navigation::LAYOUT_TABS) {
         $cssClass[] = static::CSS_CLASS_NAV_TABS;
     } elseif ($this->navigation->getLayout() === Navigation::LAYOUT_DROPDOWN) {
         $cssClass[] = static::CSS_CLASS_NAV_DROPDOWN;
     }
     return '<ul class="' . join(' ', $cssClass) . '">';
 }