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static $counter;
static $childrenCount;

    function display_element($element, &$children_elements, $max_depth, $depth = 0, $args, &$output)
    {
        $element->hasChildren = isset($children_elements[$element->ID]) && !empty($children_elements[$element->ID]);
        $element->childrenCount = count( $children_elements[$element->ID] );

        return parent::display_element($element, $children_elements, $max_depth, $depth, $args, $output);
    }

function start_el(&$output, $item, $depth, $args) {
		if( $depth )
		{
			$this->counter += 1;
		}else
		{
			$this->counter = 0;
			$this->childrenCount = $item->childrenCount;
		}
}

	function end_el(&$output, $item, $depth) {
		$output .= "</li>\n";

		$theme_location = 'primary';
		$theme_locations = get_nav_menu_locations();
		$menu_obj = get_term( $theme_locations[$theme_location], 'nav_menu' );

		if( $this->counter%5 == 0 and $this->counter != $this->childrenCount and $this->counter != 0 and $this->counter != $menu_obj->count )
		{
			$output .= '</ul><ul>';
		}
	}