ImmutableMap.Builder<String, Integer> builder = new Builder<>();
    final StringHolder holder = new StringHolder();
    holder.string = "one";
    Entry<String, Integer> entry =
        new AbstractMapEntry<String, Integer>() {
          @Override
          public String getKey() {
            return holder.string;
          }

          @Override
          public Integer getValue() {

        }

        @Override
        public Entry<K, ImmutableSet<V>> next() {
          final Entry<K, V> backingEntry = backingIterator.next();
          return new AbstractMapEntry<K, ImmutableSet<V>>() {
            @Override
            public K getKey() {
              return backingEntry.getKey();
            }

            @Override

   */
  static <K extends @Nullable Object, V extends @Nullable Object> Entry<K, V> unmodifiableEntry(
      final Entry<? extends K, ? extends V> entry) {
    checkNotNull(entry);
    return new AbstractMapEntry<K, V>() {
      @Override
      @ParametricNullness
      public K getKey() {
        return entry.getKey();
      }

      @Override
      @ParametricNullness