The teachpack implements an elevator simulator.
It displays an eight-floor elevator and accepts mouse clicks from the user, which are translated into service demands for the elevator.
Example: Define a function that consumes the current state of the elevator (three arguments) and returns a number between 1 and 8. Here is a non-sensical definition:
(define (controller x y z) 7)
It moves the elevator once, to the 7th floor.