exception Already_forced
let nondef () = lazy ( raise Lazy.Undefined )
let set (dest:'a Lazy.t) (src:'a Lazy.t) =
    (if	Obj.tag dobj <> Obj.lazy_tag ||
	    Obj.tag sobj <> Obj.lazy_tag
	then raise Already_forced
	else Obj.set_field dobj 0 (Obj.field sobj 0))
    where {dobj = Obj.repr dest and sobj = Obj.repr src}