module Nat = 

 struct

 type nat = Suc of nat | Zero_nat;;

 end;; (*struct Nat*)

 module Codegen = 

 struct

 type 'a null = {null : 'a};;

 let null _A = _A.null;;

 let rec head _A = function x :: xs -> x

                   | [] -> null _A;;

 let rec null_option = None;;

 let null_optiona () = ({null = null_option} : ('a option) null);;

 let rec dummy

   = head (null_optiona ()) [Some (Nat.Suc Nat.Zero_nat); None];;

 end;; (*struct Codegen*)