Nette 2.1.1 addRule with my function

How to call my function in Nette 2.1.1 in rule in form?

        $form->addText( 'numberx', 'Number:' )
                ->addRule('divisibilityValidator', 'Number muss be div %d.', 8);
...

    function divisibilityValidator($item, $arg)
    {
        echo "xxx";
        return $item->value % $arg === 0;
    }

Error: Unknown operation ‘divisibilityValidator’ for control ‘numberx’