module main {
    // Define the state of each traffic light
    type state_t = enum { green, red };

    // State variables for each traffic light
    var traffic_light1 : state_t;
    var traffic_light2 : state_t;

    // Initialization of traffic lights
    init {
        traffic_light1 = red;
        traffic_light2 = green;
    }

    // Transition function based on actions A and B
    procedure next_state(action: boolean)
    modifies traffic_light1, traffic_light2;
    {
        if (action == true) { // 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;
            }
        }
    }

    // Define next state relations
    next {
        // Non-deterministically choose between action A and B
        havoc action : boolean;
        call next_state(action);
    }

    // Check for deadlock: no possible action leads to a state change
    invariant no_deadlock :
        !((traffic_light1 == red && traffic_light2 == red) || (traffic_light1 == green && traffic_light2 == green));

    // Control block for verification
    control {
        v = unroll(10); // Check up to 10 steps
        check;
        print_results;
    }
}
