code: function indexPartLevels(part, index) {
      var baseLevel = String(index + 1);
      var articles = part.getArticles();
  
      articles = articles.map(function(inner, i) {
          return indexArticleLevels(inner, baseLevel + '.' + (i + 1));
      });
  
      return part.merge({
          level: baseLevel,
          articles: articles
      });
  }
docs: Index levels in a part
  
  @param {Part}
  @param {Number} index
  @return {Part}