protected function whyCantCopyVersion()
    {
        $originalPage = $this->getSingleOriginalPage();
        if ($originalPage === null) {
            return t("It's possible to copy just one page version at a time.");
        }
        if ($originalPage->isExternalLink()) {
            return t("It's not possible to copy the page version of an external URL.");
        }
        if ($originalPage->isAliasPage()) {
            return t("It's not possible to copy the page version of aliases.");
        }
        $destinationPage = $this->getDestinationPage();
        if ($destinationPage->getCollectionID() == $originalPage->getCollectionID()) {
            return t("It's not possible to copy the page version of a page to the page itself.");
        }
        $pc = new Checker($destinationPage);
        if (!$pc->canWrite()) {
            return t('You don\'t have the permission to edit the contents of "%s".', $destinationPage->getCollectionName());
        }

        return '';
    }