docs: function prepareAssets(output) {
      var book = output.getBook();
      var pages = output.getPages();
      var logger = output.getLogger();
  
      return Parse.listAssets(book, pages)
      .then(function(assets) {
          logger.info.ln('found', assets.size, 'asset files');
  
          return output.set('assets', assets);
      });
  }