'Invoked to enroll a user for this interface.  If already enrolled
an error is raised.'

def enroll(self, user):

    if (self.authenticator is None):
        self.authenticator = Authenticator.objects.create(user=user, type=self.type, config=self.config)
    else:
        if (not self.allow_multi_enrollment):
            raise Authenticator.AlreadyEnrolled()
        self.authenticator.config = self.config
        self.authenticator.save()
