'Add the given user to this service.
This is an internal method intended to be overridden by
L{WordsRealm} subclasses, not called by external code.
@type user: L{IUser}
@rtype: L{twisted.internet.defer.Deferred}
@return: A Deferred which fires with L{None} when the user is
added, or which fails with
L{twisted.words.ewords.DuplicateUser} if a user with the
same name exists already.'

def addUser(self, user):

    raise NotImplementedError
