MutableClassToInstanceMap::checkedEntry);
      }

      @Override
      public Iterator<Entry<Class<? extends @NonNull B>, B>> iterator() {
        return new TransformedIterator<
            Entry<Class<? extends @NonNull B>, B>, Entry<Class<? extends @NonNull B>, B>>(
            delegate().iterator()) {
          @Override
          Entry<Class<? extends @NonNull B>, B> transform(

      super(delegate, mutex);
    }

    @Override
    public Iterator<Map.Entry<K, Collection<V>>> iterator() {
      // Must be manually synchronized.
      return new TransformedIterator<Map.Entry<K, Collection<V>>, Map.Entry<K, Collection<V>>>(
          super.iterator()) {
        @Override
        Map.Entry<K, Collection<V>> transform(Map.Entry<K, Collection<V>> entry) {

      super(delegate, mutex);
    }

    @Override
    public Iterator<Map.Entry<K, Collection<V>>> iterator() {
      // Must be manually synchronized.
      return new TransformedIterator<Map.Entry<K, Collection<V>>, Map.Entry<K, Collection<V>>>(
          super.iterator()) {
        @Override
        Map.Entry<K, Collection<V>> transform(Map.Entry<K, Collection<V>> entry) {

  public static <F extends @Nullable Object, T extends @Nullable Object> Iterator<T> transform(
      Iterator<F> fromIterator, Function<? super F, ? extends T> function) {
    checkNotNull(function);
    return new TransformedIterator<F, T>(fromIterator) {
      @ParametricNullness
      @Override
      T transform(@ParametricNullness F from) {
        return function.apply(from);
      }
    };
  }

  /**

  public static <F extends @Nullable Object, T extends @Nullable Object> Iterator<T> transform(
      Iterator<F> fromIterator, Function<? super F, ? extends T> function) {
    checkNotNull(function);
    return new TransformedIterator<F, T>(fromIterator) {
      @ParametricNullness
      @Override
      T transform(@ParametricNullness F from) {
        return function.apply(from);
      }
    };
  }

  /**

    Keys(Multimap<K, V> multimap) {
      this.multimap = multimap;
    }

    @Override
    Iterator<Multiset.Entry<K>> entryIterator() {
      return new TransformedIterator<Map.Entry<K, Collection<V>>, Multiset.Entry<K>>(
          multimap.asMap().entrySet().iterator()) {
        @Override
        Multiset.Entry<K> transform(Map.Entry<K, Collection<V>> backingEntry) {

    Keys(Multimap<K, V> multimap) {
      this.multimap = multimap;
    }

    @Override
    Iterator<Multiset.Entry<K>> entryIterator() {
      return new TransformedIterator<Map.Entry<K, Collection<V>>, Multiset.Entry<K>>(
          multimap.asMap().entrySet().iterator()) {
        @Override
        Multiset.Entry<K> transform(Map.Entry<K, Collection<V>> backingEntry) {