def tiddlytag(tg):
    """Format the tags for tiddlywiki5;   
    when the tag includes white space, it is surrounded by '[[' ']]'.

    :param str tg: A tag.
    :return: Decorated tag.
    """
    if ' ' in tg:
        return '[[' + tg + ']]'   
    else:
        return tg