module Accumulator(event bool ?nickel,?dime,?gum,int credit) {
    loop {
       event int{31} nickel_amount, dime_amount, gum_amount;
       if(nickel) nickel_amount = 5; else nickel_amount=0;
       if(dime) dime_amount=10; else dime_amount=0;
       if(gum) gum_amount=-15; else gum_amount=0;
       next(credit) = credit + nickel_amount + dime_amount + gum_amount;
       pause;
    }
}