code: private BitSet findInclusiveLevels(int colIdToFind) {
      if (parent == null) {
        return null;
      }
      if (parent.getColId() == colIdToFind) {
        return inclusiveLevels;
      }
      return parent.findInclusiveLevels(colIdToFind);
    }
docs: Find the set of levels for a particular categorical column that can reach this node.
  A null return value implies the full set (i.e. every level).
  @param colIdToFind Column id to find
  @return Set of levels