* Equivalence<? super T> below, no type could satisfy both that bound and T's own
     * bound. With this type, they have some overlap: in our example, Equivalence<Number>
     * and Equivalence<Object>.
     */
    private final Equivalence<? super @NonNull T> equivalence;

    @ParametricNullness private final T reference;

     * Equivalence<? super T> below, no type could satisfy both that bound and T's own
     * bound. With this type, they have some overlap: in our example, Equivalence<Number>
     * and Equivalence<Object>.
     */
    private final Equivalence<? super @NonNull T> equivalence;

    @ParametricNullness private final T reference;

      return input.length();
    }
  }

  private static final Equivalence<String> LENGTH_EQUIVALENCE =
      Equivalence.equals().onResultOf(LengthFunction.INSTANCE);

  public void testWrap() {
    new EqualsTester()
        .addEqualityGroup(
            LENGTH_EQUIVALENCE.wrap("hello"),
            LENGTH_EQUIVALENCE.wrap("hello"),
            LENGTH_EQUIVALENCE.wrap("world"))

  // actually implement BiPredicate, and CollectorTests expects a BiPredicate.
  static <C, E extends @Nullable Object, R extends Iterable<E>>
      BiPredicate<C, C> pairwiseOnResultOf(Function<C, R> arg) {
    Equivalence<C> equivalence = Equivalence.equals().<E>pairwise().onResultOf(arg);
    return equivalence::equivalent;
  }

      return input.length();
    }
  }

  private static final Equivalence<String> LENGTH_EQUIVALENCE =
      Equivalence.equals().onResultOf(LengthFunction.INSTANCE);

  public void testWrap() {
    new EqualsTester()
        .addEqualityGroup(
            LENGTH_EQUIVALENCE.wrap("hello"),
            LENGTH_EQUIVALENCE.wrap("hello"),
            LENGTH_EQUIVALENCE.wrap("world"))

        Maps.toImmutableEnumMap(Entry::getKey, Entry::getValue);
    Equivalence<ImmutableMap<AnEnum, Integer>> equivalence =
        Equivalence.equals().<Entry<AnEnum, Integer>>pairwise().onResultOf(ImmutableMap::entrySet);
    CollectorTester.of(collector, equivalence)
        .expectCollects(
            ImmutableMap.of(AnEnum.A, 1, AnEnum.C, 2, AnEnum.E, 3),

  private CollectorTester(
      Collector<T, A, R> collector, BiPredicate<? super R, ? super R> equivalence) {
    this.collector = checkNotNull(collector);
    this.equivalence = checkNotNull(equivalence);
  }

  /**
   * Different orderings for combining the elements of an input array, which must all produce the
   * same result.
   */
  enum CollectStrategy {

  }

  public void testToImmutableSet() {
    Collector<String, ?, ImmutableSet<String>> collector = ImmutableSet.toImmutableSet();
    Equivalence<ImmutableSet<String>> equivalence =
        Equivalence.equals().onResultOf(ImmutableSet::asList);
    CollectorTester.of(collector, equivalence)
        .expectCollects(ImmutableSet.of("a", "b", "c", "d"), "a", "b", "a", "c", "b", "b", "d");
  }

  @CanIgnoreReturnValue
  CacheBuilder<K, V> keyEquivalence(Equivalence<Object> equivalence) {
    checkState(keyEquivalence == null, "key equivalence was already set to %s", keyEquivalence);
    keyEquivalence = checkNotNull(equivalence);
    return this;
  }

  Equivalence<Object> getKeyEquivalence() {
    return MoreObjects.firstNonNull(keyEquivalence, getKeyStrength().defaultEquivalence());
  }

  /**

     */

    STRONG {
      @Override
      Equivalence<Object> defaultEquivalence() {
        return Equivalence.equals();
      }
    },

    SOFT {
      @Override
      Equivalence<Object> defaultEquivalence() {
        return Equivalence.identity();
      }
    },

    WEAK {
      @Override
      Equivalence<Object> defaultEquivalence() {
        return Equivalence.identity();