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 Daniel Burke
 * SPDX-License-Identifier: Apache-2.0
 */

`timescale 1ns / 1ps
`default_nettype none

module tt_um_drburke3_top (
    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,      // will go high when the design is enabled
    input  wire       clk,      // clock
    input  wire       rst_n     // reset_n - low to reset
);

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

sklansky_adder_8bit diff_1(
.a      (ui_in[7:0]),   // input a
.sum    (uo_out[7:0]),  // addition out
.b      (uio_in[7:0]),  // input b
.enable (ena),          // will go high when the design is enabled
.clock  (clk),          // clock
.reset_n  (rst_n)       // reset 
);

endmodule


//////////////////////////////////////////////////////////////////////////////////
// Company: Berkeley Neuromorphic
// Engineer: Daniel Burke
// 
// Create Date: 09/14/2022 10:16:27 AM
// Design Name: 
// Module Name: sklansky_adder_8bit
// Project Name: 
// Target Devices: 
// Tool Versions: 
// Description: 8-bit generated fast adder with black, gray, and generate-propagate included at bottom
//		naming changed, truncated at 8-bits, and 'multiple packed array wires' error fixed.
// 
// Dependencies: self-contained
// 
// Revision:
// Revision 0.01 - File Created
// Additional Comments:
// from fluffybird2323 generator
// https://github.com/fluffybird2323/n-bit-sklansky-adder-code-generator-notebook/blob/master/README.Black
//////////////////////////////////////////////////////////////////////////////////


/* Generated 8 bit sklansky adder 
In this adder, binary tree of propagate and generate 
cells will first simultaneously generate all the carries, Cin. 
It builds recursively 2-bit adders then 4-bit adders, 8-bit adders, 
16-bit adder and so on by abutting each time two smaller adders. 
*/

module sklansky_adder_8bit(a,b,sum,enable,clock,reset_n);
input [7:0] a;
input [7:0] b;
output reg [7:0] sum;
input  enable;
input  clock;
input  reset_n;

// declare array wires
wire [8:0] g [8:0];
wire [8:0] p [8:0];

assign g[0][0]=1'b0;
assign p[0][0]=1'b0;
	
	generate_propagate GeneratePropagate_00(a[0],b[0],g[1][1],p[1][1]);
	generate_propagate GeneratePropagate_01(a[1],b[1],g[2][2],p[2][2]);
    generate_propagate GeneratePropagate_02(a[2],b[2],g[3][3],p[3][3]);
	generate_propagate GeneratePropagate_03(a[3],b[3],g[4][4],p[4][4]);
	generate_propagate GeneratePropagate_04(a[4],b[4],g[5][5],p[5][5]);
	generate_propagate GeneratePropagate_05(a[5],b[5],g[6][6],p[6][6]);
	generate_propagate GeneratePropagate_06(a[6],b[6],g[7][7],p[7][7]);
	generate_propagate GeneratePropagate_07(a[7],b[7],g[8][8],p[8][8]);

/// nomenclature is first number is clock level (1 is highest)
/// nomenclature is second number is bit position (0 is rightmost)

/// Level 1:	
	gray_cell GrayCell_1_1(g[1][1],p[1][1],g[0][0],g[1][0]);
	black_cell BlackCell_1_3(g[3][3],p[3][3],g[2][2],p[2][2],g[3][2],p[3][2]);
	black_cell BlackCell_1_5(g[5][5],p[5][5],g[4][4],p[4][4],g[5][4],p[5][4]);
	black_cell BlackCell_1_7(g[7][7],p[7][7],g[6][6],p[6][6],g[7][6],p[7][6]);
	
/// Level 2:
	gray_cell grayCell_2_2(g[2][2],p[2][2],g[1][0],g[2][0]);
	gray_cell grayCell_2_3(g[3][2],p[3][2],g[1][0],g[3][0]);
	black_cell BlackCell_2_6(g[6][6],p[6][6],g[5][4],p[5][4],g[6][4],p[6][4]);
	black_cell BlackCell_2_7(g[7][6],p[7][6],g[5][4],p[5][4],g[7][4],p[7][4]);
	
/// Level 3:
	gray_cell GrayCell_3_4(g[4][4],p[4][4],g[3][0],g[4][0]);
	gray_cell grayCell_3_5(g[5][4],p[5][4],g[3][0],g[5][0]);
	gray_cell grayCell_3_6(g[6][4],p[6][4],g[3][0],g[6][0]);
	gray_cell grayCell_3_7(g[7][4],p[7][4],g[3][0],g[7][0]);
	
/// Level 4: (if carry out needed)
///	gray cell_4_8(g[8][8],p[8][8],g[7][0],g[8][0]);

always @(posedge clock)
begin
	if(reset_n == 1'b0)
        begin
            sum <= 8'b00000000;      
        end 
	else if(enable == 1'b1)
        begin
           	sum[0] <= g[0][0]^p[1][1];
		    sum[1] <= g[1][0]^p[2][2];
	       	sum[2] <= g[2][0]^p[3][3];
	      	sum[3] <= g[3][0]^p[4][4];
	       	sum[4] <= g[4][0]^p[5][5];
	       	sum[5] <= g[5][0]^p[6][6];
	       	sum[6] <= g[6][0]^p[7][7];
	       	sum[7] <= g[7][0]^p[8][8];   
    end 
end
	
endmodule

// Generate Propagate code
module generate_propagate(A,B,G,P);
input A,B;
output G,P;
assign G = A&B;
assign P = A^B;
endmodule

// Gray module code
module gray_cell(G4_3,P4_3,G2_2,G4_2);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule



// Black module code
module black_cell(G6_8,P6_8,G7_10,P7_10,G6_10,P6_10);
  input G6_8,P6_8,G7_10,P7_10;
  output G6_10,P6_10;
  wire signal;
  assign signal = P6_8 & G7_10;
  assign G6_10=signal | G6_8;
  assign P6_10=P6_8 & P7_10;
endmodule


Reference solution:
rtl/generated.v:   input G4_3,P4_3,G2_2;
  output G4_2;
  wire signal;
  assign signal = P4_3 & G2_2;
  assign G4_2=signal | G4_3;