code: function indexArticleLevels(article, baseLevel) {
      baseLevel = baseLevel || article.getLevel();
      var articles = article.getArticles();
  
      articles = articles.map(function(inner, i) {
          return indexArticleLevels(inner, baseLevel + '.' + (i + 1));
      });
  
      return article.merge({
          level: baseLevel,
          articles: articles
      });
  }
docs: Index levels in an article tree
  
  @param {Article}
  @param {String} baseLevel
  @return {Article}