'Return the URL for the combined login/registration page.'

@property
def url(self):

    url = '{base}/{login_or_register}'.format(base=BASE_URL, login_or_register=self._start_page)
    if (self._course_id is not None):
        url += '?{params}'.format(params=urlencode({'course_id': self._course_id, 'enrollment_action': 'enroll'}))
    return url
