def add(message):
    """Add the message to the store.

        :param message: An email.message.Message instance containing at least
            a unique Message-ID header.  The message will be given an
            Message-ID-Hash header, overriding any existing such header.  If
            the message already exists in the store, it is not added again.
        :returns: The calculated Message-ID-Hash header or None if the message
            already exists in the store.
        :rtype: str or None
        :raises ValueError: if the message is missing a Message-ID header.
        """
