| Unregister_client of client_id (* client id *)
| List_tutors of client_id (* client_id *)
| Subscribe of client_id * tutor_id list (* client id, tutor id list *)
| Unregister_client of client_id (* client id *)
| List_tutors of client_id (* client_id *)
| Subscribe of client_id * tutor_id list (* client id, tutor id list *)