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 Your Name
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

module tt_um_c1_array_mult_structural (
    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
);
    wire[3:0] m=ui_in[7:4];
    wire[3:0] q=ui_in[3:0];
    wire[7:0] p;
    wire[3:0] w1, w2, w3, w4, partial1, partial2, partial3;
    wire[2:0] C;
    
    assign w1 = {m[3]&q[0], m[2]&q[0], m[1]&q[0], m[0]&q[0]};
    assign w2 = {m[3]&q[1], m[2]&q[1], m[1]&q[1], m[0]&q[1]};
    assign w3 = {m[3]&q[2], m[2]&q[2], m[1]&q[2], m[0]&q[2]};
    assign w4 = {m[3]&q[3], m[2]&q[3], m[1]&q[3], m[0]&q[3]};
    
    add_4bit stage0 (w2,{1'b0, w1[3:1]},partial1,C[0]);
    add_4bit stage1 (w3,{C[0], partial1[3:1]},partial2,C[1]);
    add_4bit stage2 (w4,{C[1], partial2[3:1]},partial3,C[2]);
    
    assign p = {C[2], partial3, partial2[0], partial1[0],w1[0]};
  // All output pins must be assigned. If not used, assign to 0.
  assign uo_out  = p;
  assign uio_out = 0;
  assign uio_oe  = 0;

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

endmodule

module add_4bit(x,y,z,carry_out);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule



module fulladd(a,b,Cin,sum,Cout);
    input a, b, Cin;
    output sum, Cout;
    wire z1,z2,z3;
 
    assign sum = a ^ b ^ Cin;
    assign z1 = a & b;
    assign z2 = a & Cin;
    assign z3 = b & Cin;
    assign Cout = z1 | z2 | z3;
endmodule


Reference solution:
rtl/generated.v:     input [3:0] x,y;
    output [3:0] z;
    output carry_out;
    wire [3:1] C;
    
    fulladd stage0 (x[0],y[0],1'b0,z[0],C[1]);
    fulladd stage1 (x[1],y[1],C[1],z[1],C[2]);
    fulladd stage2 (x[2],y[2],C[2],z[2],C[3]);
    fulladd stage3 (x[3],y[3],C[3],z[3],carry_out);