/// <summary>
        /// Returns the iterator pointing to the first item greater than <paramref name="item"/>.
        /// </summary>
        public RedBlackTreeIterator <T> UpperBound(T item)
        {
            RedBlackTreeIterator <T> it = LowerBound(item);

            while (it.IsValid && _host.Compare(it.Current, item) == 0)
            {
                it.MoveNext();
            }
            return(it);
        }
        /// <summary>
        /// Returns the iterator pointing to the specified item, or an iterator in End state if the item is not found.
        /// </summary>
        public RedBlackTreeIterator <T> Find(T item)
        {
            RedBlackTreeIterator <T> it = LowerBound(item);

            while (it.IsValid && _host.Compare(it.Current, item) == 0)
            {
                if (_host.Equals(it.Current, item))
                {
                    return(it);
                }
                it.MoveNext();
            }
            return(default(RedBlackTreeIterator <T>));
        }