docs: function onAsset(output, asset) {
      var book = output.getBook();
      var options = output.getOptions();
      var bookFS = book.getContentFS();
  
      var outputFolder = options.get('root');
      var outputPath = path.resolve(outputFolder, asset);
  
      return fs.ensureFile(outputPath)
      .then(function() {
          return bookFS.readAsStream(asset)
          .then(function(stream) {
              return fs.writeStream(outputPath, stream);
          });
      })
      .thenResolve(output);
  }