javascript
module ABP {

  type control_t = enum { ZERO, ONE };

  var st : control_t; // current state
  var message1 : bool; // message part 1
  var message2 : bool; // message part 2
  var ack : bool; // acknowledgement

  // Sender module
  module sender {
    input send : bool; // signal to send message
    input recv_ack : bool; // acknowledgement received
    output message : bool; // message to be sent
    output control : control_t; // control bit

    init {
      message = false;
      control = ZERO;
    }

    next {
      if (st == ZERO) {
        message' = message1;
        control' = ZERO;
        if (send && recv_ack) {
          st' = ONE;
        }
      } else {
        message' = message2;
        control' = ONE;
        if (send && recv_ack) {
          st' = ZERO;
        }
      }
    }
  }

  // Receiver module
  module receiver {
    input received_message : bool; // message received
    input control_bit : control_t; // control bit received
    output ack : bool; // acknowledgement to be sent

    init {
      ack = false;
    }

    next {
      if (st == ZERO) {
        ack' = (received_message && control_bit == ZERO);
      } else {
        ack' = (received_message && control_bit == ONE);
      }
    }
  }

  // Message channel
  module message_channel {
    input send_message : bool; // message to be sent
    output received_message : bool; // received message

    init {
      received_message = false;
    }

    next {
      received_message' = send_message;
    }
  }

  // Acknowledgement channel
  module ack_channel {
    input send_ack : bool; // acknowledgement to be sent
    output received_ack : bool; // received acknowledgement

    init {
      received_ack = false;
    }

    next {
      received_ack' = send_ack;
    }
  }

  // Main module
  module main {
    var s : sender;
    var r : receiver;
    var m : message_channel;
    var a : ack_channel;

    init {
      s.message = message1;
      s.control = ZERO;
      s.st = ZERO;
      r.st = ZERO;
    }

    next {
      s.send = true;
      r.received_message = m.send_message;
      r.control_bit = s.control;
      a.send_ack = r.ack;
      m.received_message = s.message;
    }

    // Properties
    invariant alternating_control_bits: (s.control != s'.control) -> (r.ack == r'.ack);
    invariant ack_on_correct_message: r.received_message && r.control_bit == s.control -> r.ack;
    invariant resend_on_incorrect_control_bit: r.received_message && r.control_bit != s.control -> r'.ack == r.ack;
    invariant retry_on_missing_ack: s.send && !r.ack -> s'.send;
  }
}
