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) 2024 Weihua Xiao
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

module tt_um_carryskip_adder9 (
    input  wire [7:0] ui_in,    // a input
    output wire [7:0] uo_out,   // Dedicated outputs
    input  wire [7:0] uio_in,   // b input
    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
);

    wire [7:0] a, b;
    reg [7:0] sum;
    wire cin = 0; // Initial carry-in is 0

    assign a = ui_in;         // 'a' input from ui_in
    assign b = uio_in;        // 'b' input from uio_in

    // First 4-bit block (lower half)
    wire [3:0] sum_lower;
    wire c3; // Carry out from the lower block
    ripplemod ripple_lower (a[3:0], b[3:0], cin, sum_lower, c3);

    // Skip logic for lower 4-bit block
    wire p_lower = & (a[3:0] ^ b[3:0]); // Propagate signal for lower block

    // Second 4-bit block (upper half)
    wire [3:0] sum_upper;
    wire c7; // Carry out from the upper block
    wire skip_cin = p_lower ? cin : c3; // Corrected
    ripplemod ripple_upper (a[7:4], b[7:4], skip_cin, sum_upper, c7);

    // Register sum and apply reset logic
    always @(posedge clk or negedge rst_n) begin
        if (!rst_n)
            sum <= 8'b0;  // Reset sum to 0
        else
            sum <= {sum_upper, sum_lower};
    end

    assign uo_out = sum;      // Assign the sum to output
    assign uio_out = 8'b00000000;
    assign uio_oe = 8'b00000000;
endmodule

module ripplemod(a, b, cin, sum, cout);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule



module fulladd(a, b, cin, sum, cout);
    input a, b, cin;
    output sum, cout;

    assign sum = a ^ b ^ cin;
    assign cout = (a & b) | (b & cin) | (a & cin);
endmodule


Reference solution:
rtl/generated.v:     input [3:0] a, b;
    input cin;
    output [3:0] sum;
    output cout;

    wire [2:0] c;
    fulladd fa0(a[0], b[0], cin, sum[0], c[0]);
    fulladd fa1(a[1], b[1], c[0], sum[1], c[1]);
    fulladd fa2(a[2], b[2], c[1], sum[2], c[2]);
    fulladd fa3(a[3], b[3], c[2], sum[3], cout);