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();

    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();

          } else {
            return immutableEntry(range.upperBound, range);
          }
        }
      };
    }

    @Override
    Iterator<Entry<Cut<C>, Range<C>>> descendingEntryIterator() {
      Collection<Range<C>> candidates;
      if (upperBoundWindow.hasUpperBound()) {
        candidates =
            rangesByLowerBound
                .headMap(upperBoundWindow.upperEndpoint(), false)

        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() {