Add mark to the index-th character of the given string. Return the new string after applying change.
    Notice: index > 0

def add_mark_at(string, index, mark):
    """
    Add mark to the index-th character of the given string. Return the new string after applying change.
    Notice: index > 0
    """
    if index == -1:
        return string
    # Python can handle the case which index is out of range of given string
    return string[:index] + add_mark_char(string[index], mark) + string[index+1:]