CarPark - Communication Guards
Cars enter through the WEST gate and leave through the EAST gate.
The CARPARK can hold a maximum of N = 4 cars.
When CARPARK is full then cars are not permitted to enter until a car leaves
creating a space. As is the nature of carparks, cars try to enter at twice the rate that
they leave. The program uses guarded communication. Arrival events are not accepted
when the carpark is full.
Compare the behaviour of this progam with the
Bounded Semaphore program.