doc-src/IsarAdvanced/Codegen/Thy/examples/class.ocaml

Reintroduce set_interpreter for Collect and op :.
I removed them by accident when removing old code that dealt with the "set" type.
Incidentally, there is still some broken "set" code in Refute that should be fixed (see TODO in refute.ML).

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*)