moved recursion combinator to HOL/Library/Wfrec.thy -- it is so fundamental and well-known that it should survive recdef
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*)