protected function createSplitDir($split) {
    $split_dir = $this->configSplitDir . '/' . strtolower($split);
    $result = $this->taskFilesystemStack()
      ->mkdir($split_dir)
      ->run();
    if (!$result->wasSuccessful()) {
      throw new BltException("Unable to create $split_dir.");
    }
    if (!file_exists($split_dir . '/README.md')) {
      $readme = $this->twig->render('README.md.twig', [
        'name' => $split,
      ]);
      file_put_contents($split_dir . '/README.md', $readme);
    }
  }