| 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 *)
- | State_change of client_id * state (* client_id, new state *)
+ | State_change of client_id * state option (* client_id, new state *)
| Wow of client_id (* client_id *)
(* tutor -> broker *)