public function stack($icons)
    {
        if (count($icons) !== 2)
        {
            throw new \InvalidArgumentException('Expecting exactly 2 icons in the stack');
        }

        $contents = [];
        $index = 2;
        foreach ($icons as $key => $value)
        {
            $contents[] = $this->getStackIconElement($key, $value, $index);
            --$index;
        }

        return $this->html->span($contents)->addClass('fa-stack');
    }