/**
* @param string|HtmlElement $element
*
* @return $this|self
*/
public function add($element) : self
{
if (is_string($element) && $element == '---') {
$element = (new HtmlElement('<li>'))->addAttribute('role', 'separator')->addClass('divider');
} elseif (is_string($element)) {
$element = (new HtmlElement('<li>'))->setContent($element)->addClass('dropdown-header');
} else {
$element = (new HtmlContainer('<li>'))->add($element);
}
$this->ul->add($element);
return $this;
}