node dispatcher (credit:int) returns (gum:bool);
let
         /* PRE turns the program causal  */
   gum = (0 -> pre credit) >= 15;
	     /* true when ((0 -> pre credit) >= 15); */
     	 /* it is rejected by the lustre compiler */
tel;

node accumulator (nickel,dime,gum:bool) returns (credit:int);
var nickelamount,dimeamount,gumamount:int;
let
   nickelamount = if nickel then 5 else 0;
   dimeamount   = if dime then 10 else 0;
   gumamount    = if gum then -15 else 0;             
   credit = (0 -> pre credit) + nickelamount + dimeamount + gumamount; 
            /* PRE enables memory */
tel;

node vendingMachine (nickel,dime:bool) returns (gum:bool);
var credit:int;
let
   credit = accumulator(nickel, dime, gum); 
   gum = dispatcher(credit);	
tel;