def append(header, pattern, chain=None, tag=None):
    """Append the given rule to this mailing list's header match list.

        :param header: The email header to filter on.  It will be converted to
            lower case for consistency.
        :type header: string
        :param pattern: The regular expression to use.
        :type pattern: string
        :param chain: The chain to jump to, or None to use the site-wide
            antispam jump chain via the configuration.  Defaults to None.
        :type chain: string or None
        :param tag: An arbitrary value to identify a set of IHeaderMatches.
        :type tag: string or None
        :raises ValueError: if the header/pattern pair already exists for this
            mailing list.
        """
