    if (response.status == 200) {
          return response.json().then(function(json) {
            // index main articles
            json.articles.forEach(function(article) {
              fuse.add(article);
            });
            // download collections and index their articles
            return Promise.all(json.collections.map(function(collection) {
              return fetch(offsetURL(collection)).then(function(response) {
                if (response.status === 200) {
                  return r