/**
* Render a unique item element id based on depth
*
* @param string $string
*/
protected function render_item_id($string = null)
{
if (null !== $string) {
$this->__id_stack__->push($string);
print $this->element()->id($this->__id_stack__->to_array());
} else {
$this->__id_stack__->pop();
}
}