let moonFull = let v = ref false in fun () -> v := not !v; !v
;;

module type S = sig
 type t
 val x : t
 val f : t -> t
 val g : t -> t -> bool
end;;

module Weird (Top : sig end) = (struct
 type t = int
 let x = if moonFull () then 1 else 2
 let f x = x + 2
 let g x y = (3*x + 2*y) / (x - y + 1) = 7
end : S);;

module Gen (X : functor (Top : sig end) -> S) = X (struct end);;

module W1 = Gen (Weird);; (* moon full now *)
module W2 = Gen (Weird);; (* moon no longer full now *)
W1.g W1.x W2.x;;