on.mouse_press = function(x,y, mouse_button)
	if x >= X and x < X+Square_side and y >= Y and y < Y+Square_side then
		Cursor = {1, 1}
	elseif x >= X+Square_side and x < X+Square_side*2 and y >= Y and y < Y+Square_side then
		Cursor = {2, 1}
	elseif x >= X and x < X+Square_side and y >= Y+Square_side and y < Y+Square_side*2 then
		Cursor = {1, 2}
	elseif x >= X+Square_side and x < X+Square_side*2 and y >= Y+Square_side and y < Y+Square_side*2 then
		Cursor = {2, 2}
	else
		Cursor = {0, 0}  -- invalid
	end
end

Y = 200

X = 250

Cursor = {0, 0}  -- x, y coordinate

	Problem.x, Problem.y = x, y

	elseif chord:match('%d') then
		-- cursor visible
		local x,y = Cursor[1], Cursor[2]
		if x > 0 and y > 0 then
			-- it's not in the provided square
			if x ~= Problem.x or y ~= Problem.y then
				local c = Problem.data[y][x]
				local d = (string.byte(chord) - 48)
				if c == '' then
					Problem.data[y][x] = d
				elseif c < 2 then  -- not overfull
					Problem.data[y][x] = Problem.data[y][x]*10 + d
				end
			end
		end

	local x1,y1 = 250,200
	local x2,y2 = 450,400

	local x1,y1 = X,Y
	local x2,y2 = X+Square_side*2, Y+Square_side*2