File "pr4018_bad.ml", line 42, characters 11-17:
42 | let f (x : entity entity_container) = ()
                ^^^^^^
Error: This type "entity" = "< destroy_subject : id subject; entity_id : id >"
       should be an instance of type
         "< destroy_subject : < add_observer : 'a entity_container -> 'b; .. >;
           .. >
         as 'a"
       Type
         "id subject" =
           "< add_observer : (id subject, id) observer -> unit;
             notify_observers : id -> unit >"
       is not compatible with type
         "< add_observer : < destroy_subject : 'c; .. > entity_container -> 'b;
           .. >
         as 'c"
       Type "(id subject, id) observer" = "< notify : id subject -> id -> unit >"
       is not compatible with type
         "(< destroy_subject : < add_observer : 'd -> 'b; .. >; .. > as 'a)
         entity_container as 'd" =
           "< add_entity : 'a -> 'b; notify : 'a -> id -> unit >"
       The method "add_observer" has type "(id subject, id) observer -> unit",
       but the expected method type was
       "< destroy_subject : < add_observer : 'e; .. >; .. > entity_container ->
       'b as 'e"