private function setPrefix(InputInterface $input, OutputInterface $output)
    {
        $this->prefix = $input->getOption('prefix');

        if ($this->prefix === null) {
            $this->prefix = $this->getTransifexConfigValue('/prefix');

            if ($this->prefix === null) {
                throw new \RuntimeException('Error: unable to determine transifex prefix.');
            }
            $this->writelnVerbose($output, sprintf('<info>automatically using prefix: %s</info>', $this->prefix));
        }

        $this->checkValidSlug($this->prefix);
    }