module Result =
struct
  type ('a, 'b) t = Ok of 'a | Error of 'b
end
 
open Result
 
type 'a t = { contents : int list }
 
include
  (struct
     let no_siblings_of_t t =
       match t.contents with
         | [_] -> Ok (t : [< `No_siblings | `Has_siblings] t :> [`No_siblings] t)
         | _   -> Error `Has_siblings
   end : sig
     val no_siblings_of_t : [< `No_siblings | `Has_siblings] t -> ([ `No_siblings ] t, [> `Has_siblings ]) Result.t
   end)
 

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