code: function walkSummary(summary, fn) {
      var parts = summary.getParts();
  
      return Promise.forEach(parts, function(part) {
          return walkArticles(part.getArticles(), fn);
      });
  }
docs: Walk over summary and execute "fn" on each article
  
  @param {Summary} summary
  @param {Function(article)}
  @return {Promise}