module sender {
  var st : boolean;
  var message1 : boolean;

  init {
    st = false;
    message1 = false; // Initial message content
  }

  next {
    if (ack == st) {
      message1 = !message1; // Change message content for simplicity
      st = !st; // Toggle control bit
    }
  }
}

module receiver {
  var expected_st : boolean;
  var ack : boolean;

  init {
    expected_st = false;
  }

  next {
    if (message2 == expected_st) {
      ack = expected_st;
      expected_st = !expected_st;
    }
  }
}

module message_channel {
  var message2 : boolean;

  init {
    message2 = false;
  }

  next {
    // Simulate non-deterministic loss or duplication
    if (*) {
      message2 = sender.message1;
    }
  }
}

module acknowledgement_channel {
  var ack : boolean;

  init {
    ack = false;
  }

  next {
    // Simulate non-deterministic loss or duplication
    if (*) {
      ack = receiver.ack;
    }
  }
}

module main {
  instance s : sender;
  instance r : receiver;
  instance mc : message_channel;
  instance ac : acknowledgement_channel;

  init {
    // Initial setup for the whole system
  }

  next {
    // Define the system transitions
  }

  control {
    // Check properties
  }
}
