/**
* @return string
*/
public function render()
{
$icon_back = $this->icon_back instanceof Icon ? $this->icon_back->addCssClass('fa-stack-2x') : null;
$icon_front = $this->icon_front instanceof Icon ? $this->icon_front->addCssClass('fa-stack-1x') : null;
return \yii\helpers\Html::tag('span', $icon_back . $icon_front, $this->options);
}