code: public SimpleListHolder<T> getUniqueFlatUp() {
          if (uniqueUp == null) {
              uniqueUp = newUniqueSimpleListHolder(up_to_root, getSortProperty());
          }
          return uniqueUp;
      }
docs: Returns in a {@link ListHolder} all the T elements (without duplicate) that are present in the
  current node and its ancestors, up to the root node.