protected function mungeProjectYml() {
    $this->say("Merging BLT's <comment>blt.yml</comment> template with your project's <comment>blt/blt.yml</comment>...");
    // Values in the project's existing blt.yml file will be preserved and
    // not overridden.
    $repo_project_yml = $this->getConfigValue('blt.config-files.project');
    $template_project_yml = $this->getConfigValue('blt.root') . '/template/blt/blt.yml';
    $munged_yml = YamlMunge::mungeFiles($template_project_yml, $repo_project_yml);
    try {
      YamlMunge::writeFile($repo_project_yml, $munged_yml);
    }
    catch (\Exception $e) {
      throw new BltException("Could not update $repo_project_yml.");
    }
  }