protected function isValidRule($validator, $parameters, $value) {
        if (!array_key_exists($validator, $this->availableValidators)) {
            throw new ValidatorException('"'.$validator.'" not found as available validator.');
        }
        return $this->availableValidators[$validator]->isValid($value, $parameters) ?
            null : $this->availableValidators[$validator]->getInvalidDetails();
    }