}

    /**
     * Adds e to this SetBuilderImpl, returning the updated result. Only use the returned
     * SetBuilderImpl, since we may switch implementations if e.g. hash flooding is detected.
     */
    abstract SetBuilderImpl<E> add(E e);

    /** Adds all the elements from the specified SetBuilderImpl to this SetBuilderImpl. */
    final SetBuilderImpl<E> combine(SetBuilderImpl<E> other) {
      SetBuilderImpl<E> result = this;