@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());
  }

  /**

  @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());
  }

  /**

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

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

    /**
     * Returns the default equivalence strategy used to compare and hash keys or values referenced

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

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

    /**
     * Returns the default equivalence strategy used to compare and hash keys or values referenced

      }
      return builder.build();
    }

    /**
     * Copies the current contents of this set view into an existing set. This method has equivalent
     * behavior to {@code set.addAll(this)}, assuming that all the sets involved are based on the
     * same notion of equivalence.
     *
     * @return a reference to {@code set}, for convenience
     */
    // Note: S should logically extend Set<? super E> but can't due to either

      }
      return builder.build();
    }

    /**
     * Copies the current contents of this set view into an existing set. This method has equivalent
     * behavior to {@code set.addAll(this)}, assuming that all the sets involved are based on the
     * same notion of equivalence.
     *
     * @return a reference to {@code set}, for convenience
     */
    // Note: S should logically extend Set<? super E> but can't due to either

    Map<Integer, String> right = ImmutableMap.of(1, "A", 3, "F", 5, "G", 6, "Z");

    // TODO(kevinb): replace with Ascii.caseInsensitiveEquivalence() when it
    // exists
    Equivalence<String> caseInsensitiveEquivalence =
        Equivalence.equals()
            .onResultOf(
                new Function<String, String>() {
                  @Override
                  public String apply(String input) {

  /**
   * Returns the empty iterator.
   *
   * <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
   */
  static <T extends @Nullable Object> UnmodifiableIterator<T> emptyIterator() {
    return emptyListIterator();
  }

  /**
   * Returns the empty iterator.
   *
   * <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
   */

  /**
   * Returns the empty iterator.
   *
   * <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
   */
  static <T extends @Nullable Object> UnmodifiableIterator<T> emptyIterator() {
    return emptyListIterator();
  }

  /**
   * Returns the empty iterator.
   *
   * <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
   */

   */
  public CharMatcher or(CharMatcher other) {
    return new Or(this, other);
  }

  /**
   * Returns a {@code char} matcher functionally equivalent to this one, but which may be faster to
   * query than the original; your mileage may vary. Precomputation takes time and requires more