let list_remove x l = List.filter (fun x' -> x' != x) l

class observer =
  object
    method value_changed = print_string "value_changed"
  end;;

class ['a] observable =
  object
    constraint 'a = #observer
    val mutable observers : 'a list = []
    method add_observer o = observers <- o :: observers
    method remove_observer o = observers <- list_remove o observers
    method notify = List.map (fun o -> o#value_changed) observers
  end;;


let a : 'a observable = new observable;;

let b : observer = new observer;;

a#add_observer b;;

a#notify;;