module Set_once:This module implements an option ref that starts out as None, and may be set only once. If one tries to set it twice a run time error is generated.sig..end
exception Already_set
type 'a t
include Sexpable.S1
val create : unit -> 'a tval set : 'a t -> 'a -> (unit, string) Result.tval set_exn : 'a t -> 'a -> unitval get : 'a t -> 'a option