public static function exportSynonymsToTxt(array $synonyms) : string
    {
        if (empty($synonyms)) {
            throw new \InvalidArgumentException('Nothing to export!', 1502978329);
        }

        $contentLines = '';
        foreach ($synonyms as $synonymBaseWord => $synonymWords) {
            $contentLines .= $synonymBaseWord . ' => ' . implode(',', $synonymWords) . PHP_EOL;
        }
        return rtrim($contentLines);
    }