function bufferBy(observable, startingIndex=0, bufferLength=1) {
  return observable.flatMap(nodes => Rx.Observable.create(observer => {
    let index = startingIndex;
    while ((index + bufferLength) <= nodes.length) {
      observer.next(nodes.slice(index, bufferLength));
      index += bufferLength;
    }

    while (index < nodes.length - 1) {
      observer.next(nodes.slice(index, nodes.length - index));
      index += 1;
    }
  }));
}