def register(email, display_name=None):
    """Register the given email address and link it to this user.

        :param email: The text email address to register.
        :type email: str
        :param display_name: The user's display name.  If not given the empty
            string is used.
        :type display_name: str
        :return: The address object linked to the user.  If the associated
            address object already existed (unlinked to a user) then the
            `display_name` parameter is ignored.
        :rtype: `IAddress`
        :raises AddressAlreadyLinkedError: if this `IAddress` is already
            linked to another user.
        """
