private List createParentBoundablesFromVerticalSlices(List[] verticalSlices, int newLevel) {
    Assert.isTrue(verticalSlices.length > 0);
    List parentBoundables = new ArrayList();
    for (int i = 0; i < verticalSlices.length; i++) {
      parentBoundables.addAll(
            createParentBoundablesFromVerticalSlice(verticalSlices[i], newLevel));
    }
    return parentBoundables;
  }