
module main {
  type color_t = enum { red, green };

  var traffic_light1 : color_t;
  var traffic_light2 : color_t;

  init {
    traffic_light1 = red; // Initialize traffic light 1 to red
    traffic_light2 = green; // Initialize traffic light 2 to green
  }

  next {
    // Action A: traffic_light1 goes from red to green and traffic_light2 goes from green to red
    if (traffic_light1 == red && traffic_light2 == green) {
      traffic_light1' = green;
      traffic_light2' = red;
    }
    // Action B: traffic_light1 goes from green to red and traffic_light2 goes from red to green
    else if (traffic_light1 == green && traffic_light2 == red) {
      traffic_light1' = red;
      traffic_light2' = green;
    }
    // If none of the actions can be taken, retain the current state (this should never happen in this model)
    else {
      traffic_light1' = traffic_light1;
      traffic_light2' = traffic_light2;
    }
  }

  // Define a predicate to check for deadlock: a deadlock occurs if no transitions are possible
  define deadlock := (traffic_light1 == green && traffic_light2 == green) || (traffic_light1 == red && traffic_light2 == red);

  // Safety property: Check that a deadlock never occurs
  invariant no_deadlock : !deadlock;

  control {
    v = unroll(5);
    check;
    print_results;
  }
}
