lvl = 0

function chop()
	if 	turtle.detectUp() then
		turtle.digUp()

		while not turtle.up() do os.sleep(1) turtle.attackUp() end
		lvl = lvl+1
		chop()
		if not turtle.detectDown() then
			while not turtle.down() do os.sleep(1) turtle.attackDown() end
		else
			turtle.digDown()
			chop()
		end
		lvl = lvl-1
	end

	if turtle.detectDown() and lvl==0 then
		turtle.digDown()
	end

	for i=1,4 do
		if turtle.detect() then
			turtle.dig()
			while not turtle.forward() do os.sleep(1) end
			chop()
			while not turtle.back() do os.sleep(1) end  --can be stoped
		end
		turtle.turnLeft()
	end
end

chop()