Package cannot be nested

1

What does package cannot be nested mean? I got the error with this code

package {

import flash.external.ExternalInterface;

public class URLUtil    {

    protected static const WINDOW_OPEN_FUNCTION : String = "window.open";

    public static function openWindow(url : String, window : String = "_blank", features : String = "") : void {
        ExternalInterface.call(WINDOW_OPEN_FUNCTION, url, window, features);
    }

}

}

function GoHome(event:MouseEvent):void
{
URLUtil.openWindow(“http://www.mywebsitename.com/”);
}