private final Adder adder;

    FailsToForwardReturnValue(Adder adder) {
      this.adder = adder;
    }

    @Override
    public int add(int a, int b) {
      return adder.add(a, b) + 1;
    }

    @Override
    public String toString() {
      return adder.toString();
    }
  }

  private static class FailsToPropagateException implements Adder {

  private static final long serialVersionUID = 7249069246863182397L;

  /** Version of plus for use in retryUpdate */
  @Override
  final long fn(long v, long x) {
    return v + x;
  }

  /** Creates a new adder with initial sum of zero. */
  public LongAdder() {}

  /**
   * Adds the given value.
   *
   * @param x the value to add
   */
  @Override
  public void add(long x) {
    Cell[] as;
    long b, v;

  private static final long serialVersionUID = 7249069246863182397L;

  /** Version of plus for use in retryUpdate */
  @Override
  final long fn(long v, long x) {
    return v + x;
  }

  /** Creates a new adder with initial sum of zero. */
  public LongAdder() {}

  /**
   * Adds the given value.
   *
   * @param x the value to add
   */
  @Override
  public void add(long x) {
    Cell[] as;
    long b, v;

  private static final long serialVersionUID = 7249069246863182397L;

  /** Version of plus for use in retryUpdate */
  @Override
  final long fn(long v, long x) {
    return v + x;
  }

  /** Creates a new adder with initial sum of zero. */
  public LongAdder() {}

  /**
   * Adds the given value.
   *
   * @param x the value to add
   */
  @Override
  public void add(long x) {
    Cell[] as;
    long b, v;

  private static final long serialVersionUID = 7249069246863182397L;

  /** Version of plus for use in retryUpdate */
  @Override
  final long fn(long v, long x) {
    return v + x;
  }

  /** Creates a new adder with initial sum of zero. */
  public LongAdder() {}

  /**
   * Adds the given value.
   *
   * @param x the value to add
   */
  @Override
  public void add(long x) {
    Cell[] as;
    long b, v;

    BiConsumer<A, SomeEnum> adder = collector.accumulator();
    adder.accept(accumulator, SomeEnum.A);
    adder.accept(accumulator, SomeEnum.B);
    ImmutableSet<SomeEnum> set = collector.finisher().apply(accumulator);
    assertThat(set).containsExactly(SomeEnum.A, SomeEnum.B);

    // Subsequent manual manipulation of the accumulator must not affect the state of the built set
    adder.accept(accumulator, SomeEnum.C);