public function singular($key, array $vars = [])
    {
        if (isset(static::$strings[$key])) {
            $varKeys = array_keys($vars);
            foreach ($varKeys as $i => $k) {
                $varKeys[$i] = '{' . $k . '}';
            }

            return str_replace($varKeys, $vars, static::$strings[$key]);
        }

        return '';
    }