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 <K, V> ImmutableSetMultimap<K, V> copyOf(
      Multimap<? extends K, ? extends V> multimap,
      @CheckForNull Comparator<? super V> valueComparator) {
    checkNotNull(multimap); // eager for GWT
    if (multimap.isEmpty() && valueComparator == null) {
      return of();
    }

    if (multimap instanceof ImmutableSetMultimap) {

    // we try hard to make them robust in practice.  We could additionally try to add in a system
    // load timeout multiplier.  Or we could try to use a CPU time bound instead of wall clock time
    // bound.  But these ideas are harder to implement.  We do not try to detect or handle a
    // user-specified -XX:+DisableExplicitGC.
    //
    // TODO(user): Consider using
    // java/lang/management/OperatingSystemMXBean.html#getSystemLoadAverage()

   * a good policy, especially because, in order for the test to pass, the
   * parameter must be misleadingly annotated as @Nullable. I suspect that
   * we'll want to remove @Nullable, add an eager checkNotNull, and loosen up
   * testRemove_nullAllowed.
   */
  @CanIgnoreReturnValue
  @Override
  public int remove(@CheckForNull Object element, int occurrences) {
    if (occurrences == 0) {

  }

  // enum singleton pattern
  private enum ToStringFunction implements Function<Object, String> {
    INSTANCE;

    @Override
    public String apply(Object o) {
      checkNotNull(o); // eager for GWT.
      return o.toString();
    }

    @Override
    public String toString() {
      return "Functions.toStringFunction()";
    }
  }

  /**
   * Returns the identity function.
   *

  }

  private static <K, V> ImmutableSetMultimap<K, V> copyOf(
      Multimap<? extends K, ? extends V> multimap,
      @CheckForNull Comparator<? super V> valueComparator) {
    checkNotNull(multimap); // eager for GWT
    if (multimap.isEmpty() && valueComparator == null) {
      return of();
    }

    if (multimap instanceof ImmutableSetMultimap) {

    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 {

    // we try hard to make them robust in practice.  We could additionally try to add in a system
    // load timeout multiplier.  Or we could try to use a CPU time bound instead of wall clock time
    // bound.  But these ideas are harder to implement.  We do not try to detect or handle a
    // user-specified -XX:+DisableExplicitGC.
    //
    // TODO(user): Consider using
    // java/lang/management/OperatingSystemMXBean.html#getSystemLoadAverage()

  }

  // enum singleton pattern
  private enum ToStringFunction implements Function<Object, String> {
    INSTANCE;

    @Override
    public String apply(Object o) {
      checkNotNull(o); // eager for GWT.
      return o.toString();
    }

    @Override
    public String toString() {
      return "Functions.toStringFunction()";
    }
  }

  /**
   * Returns the identity function.
   *

   * a good policy, especially because, in order for the test to pass, the
   * parameter must be misleadingly annotated as @Nullable. I suspect that
   * we'll want to remove @Nullable, add an eager checkNotNull, and loosen up
   * testRemove_nullAllowed.
   */
  @CanIgnoreReturnValue
  @Override
  public int remove(@CheckForNull Object element, int occurrences) {
    if (occurrences == 0) {