code: public SimpleListHolder<T> getUniqueFlatDown() {
          if (uniqueDown == null) {
              uniqueDown = newUniqueSimpleListHolder(down_to_leave, getSortProperty());
          }
          return uniqueDown;
      }
docs: Returns in a {@link ListHolder} all the T elements (without duplicates) that are present in the
  current node and all its descendants.