public function sku(array $values)
    {
        $count = count($values);

        $existing = array_filter($values, function ($sku) {
            $sku = $this->sku->get(array('sku' => $sku));
            return !empty($sku);
        });

        if ($count != count($existing)) {
            return $this->translation->text('@name is unavailable', array(
                '@name' => $this->translation->text('SKU')));
        }

        return true;
    }