*
   * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
   * multiple multisets in series.
   *
   * @since 2.0
   */
  public static class Builder<E> extends ImmutableCollection.Builder<E> {
    /*
     * `contents` is null only for instances of the subclass, ImmutableSortedMultiset.Builder. That
     * subclass overrides all the methods that access it here. Thus, all the methods here can safely