code: function walkArticles(articles, fn) {
      return Promise.forEach(articles, function(article) {
          return Promise(fn(article))
          .then(function() {
              return walkArticles(article.getArticles(), fn);
          });
      });
  }
docs: Walk over a list of articles
  
  @param {List<Article>} articles
  @param {Function(article)}
  @return {Promise}