protected function createSourceExtractors(
        InputInterface $input,
        OutputInterface $output,
        $config,
        CacheInterface $cachePool
    ) {
        // Remark: a plugin system would be really nice here, so others could simply hook themselves into the checking.
        $extractors = [];
        foreach ([
                'bower'     => AuthorExtractor\BowerAuthorExtractor::class,
                'composer'  => AuthorExtractor\ComposerAuthorExtractor::class,
                'packages'  => AuthorExtractor\NodeAuthorExtractor::class,
                'php-files' => AuthorExtractor\PhpDocAuthorExtractor::class,
            ] as $option => $class) {
            if ($input->getOption($option)) {
                $extractors[$option] = new $class($config, $output, $cachePool);
            }
        }

        return $extractors;
    }