(* .ml *)
let no_siblings_of_t t =
  match t.contents with
    | [_] -> Ok t
    | _   -> Error `Has_siblings
 
(* .mli *)
val no_siblings_of_t : 'a t -> ([ `No_siblings ] t, [> `Has_siblings ]) Result.t
 
(* error *)
Error: The implementation robj.ml does not match the interface robj.cmi:
       Values do not match:
         val no_siblings_of_t :
           [ `No_siblings ] t ->
           ([ `No_siblings ] t, [> `Has_siblings ]) Core.Std._result
       is not included in
         val no_siblings_of_t :
           'a t -> ([ `No_siblings ] t, [> `Has_siblings ]) Core.Std.Result.t
 

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