return Iterators.<@Nullable Entry<K, V>>getNext(descendingEntryIterator(), null);
  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return Iterators.pollNext(entryIterator());
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {
    return Iterators.pollNext(descendingEntryIterator());
  }

  @Override
  @ParametricNullness
  public K firstKey() {

    return Iterators.<@Nullable Entry<K, V>>getNext(descendingEntryIterator(), null);
  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return Iterators.pollNext(entryIterator());
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {
    return Iterators.pollNext(descendingEntryIterator());
  }

  @Override
  @ParametricNullness
  public K firstKey() {

    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  public @Nullable Entry<E> lastEntry() {
    Iterator<Entry<E>> entryIterator = descendingEntryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  public @Nullable Entry<E> pollFirstEntry() {
    Iterator<Entry<E>> entryIterator = entryIterator();

        setCount(prevEntry.getElement(), 0);
        prevEntry = null;
      }
    };
  }

  @Override
  Iterator<Entry<E>> descendingEntryIterator() {
    return new Iterator<Entry<E>>() {
      @Nullable AvlNode<E> current = lastNode();
      @Nullable Entry<E> prevEntry = null;

      @Override
      public boolean hasNext() {

        setCount(prevEntry.getElement(), 0);
        prevEntry = null;
      }
    };
  }

  @Override
  Iterator<Entry<E>> descendingEntryIterator() {
    return new Iterator<Entry<E>>() {
      @Nullable AvlNode<E> current = lastNode();
      @Nullable Entry<E> prevEntry = null;

      @Override
      public boolean hasNext() {

      set.clear();
    }

    @Override
    Iterator<Entry<K, V>> entryIterator() {
      return asMapEntryIterator(set, function);
    }

    @Override
    Iterator<Entry<K, V>> descendingEntryIterator() {
      return descendingMap().entrySet().iterator();
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return removeOnlyNavigableSet(set);
    }

    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
      set.forEach(k -> action.accept(k, function.apply(k)));
    }

    @Override
    Iterator<Entry<K, V>> descendingEntryIterator() {
      return descendingMap().entrySet().iterator();
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return removeOnlyNavigableSet(set);
    }