Super Simple Vending Machine
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Our vending machine takes two quarters
and waits for the Dispense button to be
pressed. It doesn't make change. It can
only dispense one drink. There is a coin
return button which may be pressed at
any time.

So,

Inputs are,

   Quarter sensor ->
      Activated when a quarter passes by it.
      It doesn't count quarters. It only says
      a quarter has been deposited.
  Coin return button ->
     Returns any deposited coins. May be pressed
     at any time.
  Dispense button ->
    Will dispense our only drink type
     when pressed.

Actions are,
   Open the coin return gate momentarilly ->
      Returns the deposited coins.
  Dispense drink ->
    Open the dispense gate momentarilly ->
      Gives the drink to the customer.

Our machine is simple .. No lights, no change,
only a single drink. If they put too many quarters
we keep the extra; we act like they just deposited
two quarters.

Remember your FSM *cannot* have global state,
i.e. counters.