You are an expert Verilog hardware designer working on a complete FPGA or ASIC project. You are given the full source code of a Verilog project. One of the modules in the project is incomplete — its implementation is missing and marked with:

// >>> Module Implementation Begin

// <<< Module Implementation End

Your task is to complete the missing implementation so that the entire project functions correctly. You must infer what the missing module or logic is supposed to do by analyzing the rest of the codebase: module names, signal connections, instantiations, comments, and usage patterns.

Follow these rules:

1. Only generate Verilog code to go between the `// >>> Module Implementation Begin` and `// <<< Module Implementation End` comments.
2. Use Verilog-2001 syntax. Ensure the code is synthesizable.
3. Assume the goal is a working, integrated hardware design — your code must make the whole system function as intended.
4. Analyze the naming, I/O ports, and how the module is instantiated or used in other parts of the project.
5. Implement correct behavior based on design context. Do not guess randomly — reason it out.
6. Avoid vendor-specific features unless explicitly required.
7. Do not output comments, explanation, or anything outside the required code region unless explicitly asked.

Your output should contain **only the Verilog code** that fits inside the marked region.

/*
 * Copyright (c) Krishna Subramanian <https://github.com/mongrelgem>
 * Modified in 2024 by Juan Gil & Leyang Wang
 * 
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

module FA(output wire sum,
          output wire cout,
          input wire a,
          input wire b,
          input wire cin);
  wire w0, w1, w2;
  
  assign w0 = a ^ b;
  assign sum = w0 ^ cin;

  assign w1 = w0 & cin;
  assign w2 = a & b;
  assign cout = w1 | w2;
  
endmodule

// Ripple Carry Adder with cin - 4 bits
module RCA4(output wire [3:0] sum,
            output wire cout,
            input wire [3:0] a,
            input wire [3:0] b,
            input wire cin);
  
  wire [3:1] c;
  
  FA fa0(sum[0], c[1], a[0], b[0], cin);
  FA fa[2:1](sum[2:1], c[3:2], a[2:1], b[2:1], c[2:1]);
  FA fa31(sum[3], cout, a[3], b[3], c[3]);
  
endmodule

module MUX2to1_w1(output wire y,
                  input wire i0,
                  input wire i1,
                  input wire s);
  wire sn;
  wire e0, e1;
  assign sn = ~s;

  assign e0 = i0 & sn;
  assign e1 = i1 & s;
  assign y = e0 | e1;
  
endmodule

module MUX2to1_w4(output wire [3:0] y,
                  input wire [3:0] i0,
                  input wire [3:0] i1,
                  input wire s);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule



module tt_um_CarrySelect8bit (
    input  wire [7:0] ui_in,    // Dedicated inputs
    output wire [7:0] uo_out,   // Dedicated outputs
    input  wire [7:0] uio_in,   // IOs: Input path
    output wire [7:0] uio_out,  // IOs: Output path
    output wire [7:0] uio_oe,   // IOs: Enable path (active high: 0=input, 1=output)
    input  wire       ena,      // always 1 when the design is powered, so you can ignore it
    input  wire       clk,      // clock
    input  wire       rst_n     // reset_n - low to reset
);

    // Internal Signals
  wire [7:0] sum, sum0, sum1, a, b;
  wire cout, cout0_0, cout0_1, cout1_0, cout1_1, c1;

          assign a[7:0] = ui_in[7:0]; 
          assign b[7:0] = uio_in[7:0];

          
          RCA4 rca0_0(sum0[3:0], cout0_0, a[3:0], b[3:0], 1'b0); //calculates 4 LSB of a + b with cin = 0
          RCA4 rca0_1(sum1[3:0], cout0_1, a[3:0], b[3:0], 1'b1); //calculates 4 LSB of a + b with cin = 1
          MUX2to1_w4 mux0_sum(sum[3:0], sum0[3:0], sum1[3:0], 1'b0); // this will always give sum0
          MUX2to1_w1 mux0_cout(c1, cout0_0, cout0_1, 1'b0); // this will always give cout0_0

          RCA4 rca1_0(sum0[7:4], cout1_0, a[7:4], b[7:4], 1'b0); //calculates 4 MSB of a + b with cin = 0
          RCA4 rca1_1(sum1[7:4], cout1_1, a[7:4], b[7:4], 1'b1); //calculates 4 MSB of a + b with cin = 0
  MUX2to1_w4 mux1_sum(sum[7:4], sum0[7:4], sum1[7:4], c1); // this will always select sum0, as c1 is always cout0_0
  MUX2to1_w1 mux1_cout(cout, cout1_0, cout1_1, c1);


    
    
  // All output pins must be assigned. If not used, assign to 0.
  assign uo_out  = sum;  // Example: ou_out is the sum of ui_in and uio_in
  assign uio_out = 0;
  assign uio_oe  = 0;

  // List all unused inputs to prevent warnings
  wire _unused = &{ena, clk, rst_n, 1'b0};

endmodule


Reference solution:
rtl/generated.v:   wire sn;
  wire [3:0] e0, e1;
  assign sn = ~s;

  assign e0[0] =  i0[0] & sn;
  assign e0[1] =  i0[1] & sn;
  assign e0[2] =  i0[2] & sn;
  assign e0[3] =  i0[3] & sn;

  assign e1[0] = i1[0] & s;
  assign e1[1] = i1[1] & s;
  assign e1[2] = i1[2] & s;
  assign e1[3] = i1[3] & s;

  assign y[0] = e0[0] | e1[0];
  assign y[1] = e0[1] | e1[1];
  assign y[2] = e0[2] | e1[2];
  assign y[3] = e0[3] | e1[3];