def permalink(mlist, msg):
    """Return the url to the message in the archive.

        This url points directly to the message in the archive.  This method
        only calculates the url, it does not actually archive the message.

        If the archiver is not web-accessible, return None.

        :param mlist: The IMailingList object.
        :param msg: The message object.
        :returns: The url string or None if the message's archive url cannot
            be calculated.
        """
