code: 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');
      }
docs: Stack two icons on top of each other. The array must contain exactly 2 elements.
  
  An element of the array can either be:
  
  - A simple string, in which case it is considered to be the icon name
  - A key/value pair, both being strings, in which case the key is the icon name and the value will be added to
  the icon css classes
  - A FontAwesomeIcon object, obtained using the icon function.
  
  @param array $icons The 2 icons to stack. First is back, second is front.
  
  @return \Spatie\Html\Elements\Span
  
  @throws \InvalidArgumentException