code: public SortedSet<T> firstSubset(SortedSet<T> set) {
          if (this.rootNodes.isEmpty() || set == null || set.isEmpty()) {
              return null;
          }
          return firstSubset(set, this.rootNodes);
      }
docs: Returns the first subset of a given set in this UBTree.
  @param set the set to search for
  @return the first subset which is found for the given set or {@code null} if there is none