/**
* Main WF Instance
*
* Ensures only one instance of WF is loaded or can be loaded.
*
* @since 6.0.0
* @static
* @see WF()
* @return Main WF instance
*/
public static function instance()
{
if (is_null(self::$_instance)) {
self::$_instance = new self();
}
return self::$_instance;
}