this.range = GeneralRange.all(comparator);
    this.header = new AvlNode<>();
    successor(header, header);
    this.rootReference = new Reference<>();
  }

  /** A function which can be summed across a subtree. */
  private enum Aggregate {
    SIZE {
      @Override
      int nodeAggregate(AvlNode<?> node) {
        return node.elemCount;
      }

      @Override

   * Elements are added in encounter order.
   *
   * <p>If the mapped elements contain duplicates (according to {@link Object#equals}), the element
   * will be added more than once, with the count summed over all appearances of the element.
   *
   * <p>Note that {@code stream.collect(toMultiset(function, e -> 1, supplier))} is equivalent to
   * {@code stream.map(function).collect(Collectors.toCollection(supplier))}.
   *