private function validateIsOrdered($method)
    {
        $innerIterator = $this->elements;
        if (!($innerIterator instanceof Iterators\IOrderedIterator)) {
            throw new PinqException(
                    'Invalid call to %s: %s::%s must be called first.',
                    $method,
                    __CLASS__,
                    'orderBy');
        }

        return $innerIterator;
    }