(* Vending machine *)

datatype Event 
   = Epsilon 
   | Nickle of Event
   | Dime of Event
   | Quarter of Event
   | Orange of Event
   | Apple of Event

(* 0 deposited *)
fun S0 Epsilon = NONE
  | S0 (Nickle C) = S1 C
  | S0 (Dime C) = S2 C
  | S0 (Quarter C) = S5 C
  | S0 (Orange C) = S0 C
  | S0 (Apple C) = S0 C
  
(* 5 deposited *)
and S1 Epsilon = NONE
  | S1 (Nickle C) = S2 C
  | S1 (Dime C) = S3 C
  | S1 (Quarter C) = S6 C
  | S1 (Orange C) = S1 C
  | S1 (Apple C) = S1 C

(* 10 deposited *)
and S2 Epsilon = NONE
  | S2 (Nickle C) = S3 C
  | S2 (Dime C) = S4 C
  | S2 (Quarter C) = S6 C
  | S2 (Orange C) = S2 C
  | S2 (Apple C) = S2 C

(* 15 deposited *)
and S3 Epsilon = NONE
  | S3 (Nickle C) = S4 C
  | S3 (Dime C) = S5 C
  | S3 (Quarter C) = S6 C
  | S3 (Orange C) = S3 C
  | S3 (Apple C) = S3 C

(* 20 deposited *)
and S4 Epsilon = NONE
  | S4 (Nickle C) = S5 C
  | S4 (Dime C) = S6 C
  | S4 (Quarter C) = S6 C
  | S4 (Orange C) = S4 C
  | S4 (Apple C) = S4 C

(* 25 deposited *)
and S5 Epsilon = NONE
  | S5 (Nickle C) = S1 C
  | S5 (Dime C) = S6 C
  | S5 (Quarter C) = S6 C
  | S5 (Orange C) = S5 C
  | S5 (Apple C) = S5 C

(* 30 deposited *)
and S6 Epsilon = NONE
  | S6 (Nickle C) = S6 C
  | S6 (Dime C) = S6 C
  | S6 (Quarter C) = S6 C
  | S6 (Orange C) = S0 C
  | S6 (Apple C) = S0 C