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

    var L1, L2, L3, L4 : color_t;

    // Initialize the traffic lights
    init {
        L1 = red; L2 = red; L3 = red; L4 = red;
    }

    // Define the next state transitions for the traffic lights
    procedure update_light(l : color_t) returns (next_color : color_t) {
        if (l == green) {
            return yellow;
        } else if (l == yellow) {
            return red;
        } else if (l == red) {
            return amber;
        } else {
            return green;
        }
    }

    // Main controller logic to update traffic lights based on the current state
    next {
        if (L1 == red && L2 == red && L3 == red && L4 == red) {
            L1' = green; L3' = green;
        } else if (L1 == green && L3 == green) {
            L1' = update_light(L1);
            L3' = update_light(L3);
        } else if (L1 == yellow && L3 == yellow) {
            L1' = update_light(L1);
            L3' = update_light(L3);
            L2' = green;
        } else if (L2 == green) {
            L2' = update_light(L2);
            L4' = green;
        } else if (L4 == green) {
            L4' = update_light(L4);
        } else {
            L1' = update_light(L1);
            L2' = update_light(L2);
            L3' = update_light(L3);
            L4' = update_light(L4);
        }
    }

    // Safety condition: No two opposite or adjacent lights should be green at the same time
    invariant safety_condition : (L1 != green || L2 != green) && (L3 != green || L4 != green);

    control {
        v = unroll(10);
        check;
        print_results;
    }
}
