public static <T extends @Nullable Object> UnmodifiableIterator<T> filter(
      Iterator<T> unfiltered, Predicate<? super T> retainIfTrue) {
    checkNotNull(unfiltered);
    checkNotNull(retainIfTrue);
    return new AbstractIterator<T>() {
      @Override
      protected @Nullable T computeNext() {
        while (unfiltered.hasNext()) {
          T element = unfiltered.next();
          if (retainIfTrue.apply(element)) {