Model a system that counts the number of cars that enter and leave a parking garage in order to keep track of how many cars are in the garage at any time.

Assume that an ArrivalDetector actor produces an event when a car arrives, and a DepartureDetector actor produces an event when a car departs. The Counter actor keeps a running count, starting from an initial value i. Each time the count changes, it produces an output event that updates a display.

Use the variable names arrival, departure and counter
