type ins =
  | Basic of char        (* a *)
  | Terminate            (* ! *)
  | Ptest of char        (* +a *)
  | Ntest of char        (* -a *)
  | Jump of int          (* #1 *)

  | Concat of ins list   (* a;b *)
  | Repeat of ins * int  (* a2 : execute a twice *)
  | Loop of ins          (* a* : repeat a infinitely *)

  | Program of char      (* X *)
  | Empty

type program = ins list

(* Natural numbers *)
type natural = Infinity | N of int

let natural_binop binop a b =
  match (a, b) with
  | N i, N j -> N (binop i j)
  | _ -> Infinity

let ( ++ ) = natural_binop ( + )
let ( -- ) = natural_binop ( - )
let ( ** ) = natural_binop ( * )
let ( // ) = natural_binop ( / )

exception Fatal_error of string
exception Ins_error of ins * string
exception Program_error of program * string