module abp {
    const DATA_SIZE : int = 2;
    type control_t = enum { C0, C1 };
    type state_t = enum { WAIT_FOR_0, WAIT_FOR_1 };

    var st : state_t;
    var message1 : int[DATA_SIZE];
    var message2 : int[DATA_SIZE];
    var ack : control_t;

    // Sender module
    module sender {
        input send : bool;
        output message : int[DATA_SIZE];
        output control : control_t;

        init {
            message = message1;
            control = C0;
        }

        next {
            if (send) {
                if (st == WAIT_FOR_0) {
                    control' = C0;
                } else {
                    control' = C1;
                }
            }
        }
    }

    // Receiver module
    module receiver {
        input receive : bool;
        input control : control_t;
        input message : int[DATA_SIZE];
        output ack : control_t;

        init {
            ack = C0;
        }

        next {
            if (receive) {
                if (control == C0 && st == WAIT_FOR_0) {
                    ack' = C0;
                } else if (control == C1 && st == WAIT_FOR_1) {
                    ack' = C1;
                }
            }
        }
    }

    // Message Channel
    module message_channel {
        input send : bool;
        input message : int[DATA_SIZE];
        output receive : bool;
        output message : int[DATA_SIZE];

        init {
            receive = false;
        }

        next {
            if (send) {
                message' = message;
                receive' = true;
            }
        }
    }

    // Acknowledgement Channel
    module ack_channel {
        input send : bool;
        input ack : control_t;
        output receive : bool;
        output ack : control_t;

        init {
            receive = false;
            ack = C0;
        }

        next {
            if (send) {
                ack' = ack;
                receive' = true;
            }
        }
    }

    // Main module orchestrating interactions between sender, receiver, message channel, and acknowledgement channel
    module main {
        var s_send, r_receive, m_send, a_send : bool;

        instance s : sender(send: s_send);
        instance r : receiver(receive: r_receive, control: s.control, message: s.message);
        instance m : message_channel(send: m_send, message: s.message);
        instance a : ack_channel(send: a_send, ack: r.ack);

        init {
            s_send = true;
            r_receive = true;
            m_send = false;
            a_send = false;
            st = WAIT_FOR_0;
        }

        next {
            s_send' = !r_receive;
            r_receive' = !s_send;
            m_send' = s_send;
            a_send' = r_receive;

            if (r_receive) {
                if (r.control == C0) {
                    st' = WAIT_FOR_1;
                } else {
                    st' = WAIT_FOR_0;
                }
            }
        }

        invariant no_corruption : forall i: [0:DATA_SIZE-1] :: m.message[i] == s.message[i];
        invariant ack_matches_control : a.ack == r.control;
    }
}
