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

exception FatalError of string