public function render($tag = null, $options = [])
{
$tag = empty($tag) ? empty($this->tag) ? static::$defaultTag : $this->tag : $tag;
$options = array_merge($this->options, $options);
$template = ArrayHelper::remove($options, 'template', '{back}{front}');
$icon_back = $this->icon_back instanceof Icon ? $this->icon_back->addCssClass(FA::$cssPrefix . '-stack-2x') : null;
$icon_front = $this->icon_front instanceof Icon ? $this->icon_front->addCssClass(FA::$cssPrefix . '-stack-1x') : null;
return Html::tag($tag, str_replace(['{back}', '{front}'], [$icon_back, $icon_front], $template), $options);
}