// Define modules for Sender (S), Receiver (R), Channel c, and Channel d.

module ChannelC
{
    // Channel c is unreliable; messages can be lost.
    // For simplicity, we model this as a nondeterministic choice to either deliver or lose the message.
    var buffer: [boolean]boolean; // Maps control bit to message presence.
    input m : boolean; // Message
    input b : boolean; // Control bit

    procedure send(m : boolean, b : boolean)
    {
        if (*) { // Nondeterministic choice
            buffer[b] = true;
        }
    }

    procedure receive(b : boolean) returns (m : boolean)
    {
        if (buffer[b]) {
            return true; // Message was delivered.
        } else {
            return false; // Message was lost.
        }
    }

    init {
        buffer[false] = false;
        buffer[true] = false;
    }
}

module ChannelD
{
    // Channel d is perfect; acknowledgments are always delivered.
    var ack : boolean; // Acknowledgment control bit

    procedure sendAck(b : boolean)
    {
        ack = b;
    }

    procedure getAck() returns (b : boolean)
    {
        return ack;
    }

    init {
        ack = false;
    }
}

module Sender
{
    var m : boolean; // Message
    var b : boolean; // Control bit
    var waitingForAck : boolean;

    procedure send(m : boolean, b : boolean)
    {
        // Implementation of sending logic, including retransmission on timeout.
    }

    procedure receiveAck(b : boolean)
    {
        // Acknowledgment handling.
    }

    init {
        b = false;
        waitingForAck = false;
    }
}

module Receiver
{
    procedure receive(m : boolean, b : boolean)
    {
        // Implementation of receiving logic.
    }

    procedure sendAck(b : boolean)
    {
        // Acknowledgment sending logic.
    }
}

// Main module to orchestrate the communication protocol.
module Main
{
    // Instances of Sender, Receiver, ChannelC, and ChannelD.
    // Logic to simulate communication, including message transmission, acknowledgment, and retransmission.
}
