code: public void addSet(final SortedSet<T> set) {
          SortedMap<T, UBNode<T>> nodes = rootNodes;
          UBNode<T> node = null;
          for (T element : set) {
              node = nodes.get(element);
              if (node == null) {
                  node = new UBNode<>(element);
                  nodes.put(element, node);
              }
              nodes = node.children();
          }
          if (node != null) {
              node.setEndSet(set);
          }
      }
docs: Adds a set of comparable objects to this UBTree.
  @param set the set of comparable objects