Help needed drawing a Sudoku board with green shoes

Hello Team,

I am planning to write a Sudoku program just for the learning experience.
I'm first trying to draw the board using *green shoes* (which I am also
learning) as my GUI toolkit.
As most people know, the most common Sudoku board is a 9x9 array. The 9x9
array is subdivided on 3x3 square. I don't want to go into details as I
know that Sudoku is already a very well know game.

To make the game easier to follow it is customary to have some kind of
division between each 3x3 square. It could be a heavy line or some space.

My problem is that I can't find a way to do this with *green shoes*. I am
convinced that it is doable as this appears to be something simple, but I
can't find the way. I already experimented with many version of my code.
Here are the last two versions.

The first version correctly draws the board but with no apparent division
between the 3x3 squares.
The second version stacks all the buttons one after the other. I thought
that the *if else end* stmt that I added could do the trick. But it did not.

Any help will be appreciated as usual.

*FIRST VERSION

require "green_shoes"
require 'rubygems'

Shoes.app :height => 666, :width => 811, :resizable => false do
   background magenta
   title (strong("Sudoku")), :size => 26, :align=>'center'
    para " "
   para " "

   @btn = Array.new(9) {Array.new(9) {123456789}}

   (0..8).each do |r|
      stack :margin => 3 do
         flow :width => 866 do
            (0..8).each do |c|
               @btn[r][c] = button "123456789", :width => 86, :height => 50
            end # End do c
         end # flow
      end # End stack
   end # End do r
end # app
*


*SECOND VERSION

*
require "green_shoes"
require 'rubygems'

Shoes.app :height => 666, :width => 811, :resizable => false do

   background magenta
   title (strong("Sudoku")), :size => 26, :align=>'center'
    para " "
   para " "

   @btn = Array.new(9) {Array.new(9) {123456789}}

   (0..8).each do |r|
      stack :margin => 3 do
         (0..8).each do |c|
            if c == 3 or c == 6
               flow :width => 866, :margin => 3 do
                  @btn[r][c] = button "123456789", :width => 86, :height =>
50
               end
            else
               flow :width => 866 do
                  @btn[r][c] = button "123456789", :width => 86, :height =>
50
               end
            end # if
         end # End do c
      end # End stack
   end # End do r
end # app

Thank you

-- 
Ruby Student