code: public Iterator<Map.Entry<K, V>> descendingIterator() {
          DescendingIterator<K, V> iterator = new DescendingIterator<>(mEnd, mStart);
          mIterators.put(iterator, false);
          return iterator;
      }
docs: Descending iterator.
  
  @return an descending iterator, which doesn't include new elements added during an
  iteration.