
module main {
    type color_t;
    const green : color_t;
    const red : color_t;
    
    // Declare state variables for each traffic light
    var traffic_light1 : color_t;
    var traffic_light2 : color_t;
    
    // Initialize the traffic lights
    init {
        traffic_light1 = red;
        traffic_light2 = green;
    }
    
    // Define actions A and B as transitions
    transition {
        // Action A: traffic_light1 goes from red to green, and traffic_light2 from green to red
        A -> (traffic_light1' = green && traffic_light2' = red)
          | (traffic_light1' = traffic_light1 && traffic_light2' = traffic_light2);
        // Action B: traffic_light1 goes from green to red, and traffic_light2 from red to green
        B -> (traffic_light1' = red && traffic_light2' = green)
          | (traffic_light1' = traffic_light1 && traffic_light2' = traffic_light2);
    }

    // Define properties to check
    define {
        green1 = (traffic_light1 = green);
        red1 = (traffic_light1 = red);
        green2 = (traffic_light2 = green);
        red2 = (traffic_light2 = red);
    }

    // Check for deadlock possibility
    // A deadlock happens when neither action is possible, meaning no change occurs anymore
    query deadlock := (forall (traffic_light1' : color_t, traffic_light2' : color_t) ::
        (traffic_light1' = traffic_light1 && traffic_light2' = traffic_light2));
}