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)) {
            return element;
          }
        }
        return endOfData();

    @Override
    public Iterator<ReferenceEntry<K, V>> iterator() {
      return new AbstractSequentialIterator<ReferenceEntry<K, V>>(peek()) {
        @Override
        protected @Nullable ReferenceEntry<K, V> computeNext(ReferenceEntry<K, V> previous) {
          ReferenceEntry<K, V> next = previous.getNextInWriteQueue();
          return (next == head) ? null : next;
        }
      };
    }
  }

  /**