code

function placeSlot(slot, dir)
 dir = dir or ''
 turtle.select(slot)
 turtle['dig' .. dir]()
 turtle['place' .. dir]()
end

for i = 1,10 do
 turtle.digDown()
 if not turtle.detect() then
  placeSlot(1)
 end
 placeSlot(2, 'Up')
 turtle.down()

 if i % 2 == 0 then
  turtle.turnLeft()
  placeSlot(3)
  turtle.turnRight()
 end
end