public static function arrow(string $direction) : string
    {
        switch ($direction) {
            case "up": 	  $output = '<span class="arrow">&uarr;</span>'; break;
            case "down":  $output = '<span class="arrow">&darr;</span>'; break;
            case "left":  $output = '<span class="arrow">&larr;</span>'; break;
            case "right": $output = '<span class="arrow">&rarr;</span>'; break;
        }

        return $output;
    }