@Override
  public void goBack() throws VcsException {
    if ((myCurrentIdx + 1) < myChunks.size()) {
      ++myCurrentIdx;
      return;
    }

    final Ref<Boolean> canNotGoBackRef = new Ref<>();
    final List<Fragment> fragments = myChunkFactory.goBack(CHUNK_SIZE, canNotGoBackRef);
    myCanNotGoBack = canNotGoBackRef.get().booleanValue();

    if (!fragments.isEmpty()) {
      // load
      ++myCurrentIdx;
      myChunks.add(fragments);
    }
  }
@Override
  public void goBack()