Pay Station Stories
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1: Buy a parking permit

   A car driver walks to the pay station to buy parking time. He
   enters several valid coins (5, 10, and 25 cents) as pyament. For
   each payment of 5 cents, he receives 2 minutes of parking time. On
   the pay station's display, he can see how much time he has
   purchased so far. Once he is satisfied with the amount of time, he
   presses the button marked "Buy." He receives a printed receipt
   stating the number of minutes of parking time he has bought. The
   display is cleared to prepare for another transaction.


2: Cancel a transaction

   A driver has entered several coins but realizes that the
   accumulated parking time exceeds what she needs. She presses the
   button marked "Cancel" and her coins are returned. The display is
   cleared to prepare for another transaction.


3: Reject illegal coin

   A driver has entered 50 cents total and the display reads "20". By
   mistake, he enters a dollar coin which is not a recognized
   coin. the paystation rejects the coin and the display is not
   updated.