*         .add(42)
   *         .build();
   * }</pre>
   *
   * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
   * multiple sets in series. Each set is a superset of the set created before it.
   *
   * @since 2.0
   */
  public static final class Builder<E> extends ImmutableSet.Builder<E> {
    private final Comparator<? super E> comparator;