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