code: public Set<SortedSet<T>> allSubsets(final SortedSet<T> set) {
          final Set<SortedSet<T>> subsets = new LinkedHashSet<>();
          allSubsets(set, this.rootNodes, subsets);
          return subsets;
      }
docs: Returns all subsets of a given set in this UBTree.
  @param set the set to search for
  @return all subsets of of the given set