function addFormRule($rule)
    {
        if (!is_callable($rule)) {
            return self::raiseError(null, QUICKFORM_INVALID_RULE, null, E_USER_WARNING, 'Callback function does not exist in HTML_QuickForm::addFormRule()', 'HTML_QuickForm_Error', true);
        }
        $this->_formRules[] = $rule;
    }