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) {