def create_user(email=None, display_name=None):
    """Create and return an `IUser`.

        :param email: The text email address for the user being created.
        :type email: str
        :param display_name: The display name of the user.
        :type display_name: str
        :return: The newly created user, with the given email address and real
            name, if given.
        :rtype: `IUser`
        :raises ExistingAddressError: when the email address is already
            registered.
        """
