/// Returns a new character sequence that is a subsequence of this sequence. The subsequence starts with the character at the specified index and ends with the character at index end - 1. The length of the returned sequence is end - start, so if start == end then an empty sequence is returned.
/// <param name="end">the end index, exclusive</param>
/// <returns>the specified subsequence</returns>
/// <exception cref="IndexOutOfRangeException"> if start or end are negative, if end is greater than length(), or if start is greater than end</exception>