module main {
    // Define the possible states for traffic lights
    type state_t = enum { red, green };

    // Variables for the states of each traffic light
    var traffic_light1 : state_t;
    var traffic_light2 : state_t;

    // Initialization
    init {
        traffic_light1 = red;
        traffic_light2 = green;
    }

    // Transition function
    procedure next_state(action : boolean)
    modifies traffic_light1, traffic_light2;
    {
        if (action) {
            // Action A
            if (traffic_light1 == red && traffic_light2 == green) {
                traffic_light1 = green;
                traffic_light2 = red;
            }
        } else {
            // Action B
            if (traffic_light1 == green && traffic_light2 == red) {
                traffic_light1 = red;
                traffic_light2 = green;
            }
        }
    }

    // Next state logic
    next {
        // Non-deterministically choose action A or B
        havoc action : boolean;
        call next_state(action);
    }

    // Safety property: Check for deadlock
    invariant no_deadlock : !(traffic_light1 == traffic_light2);

    // Control block for model checking
    control {
        v = unroll(10);
        check;
        print_results;
    }
}
