Minecraft Turtle: Stairs (Robust)

--[[
    2013 (c) psychedelixx
    Minecraft Turtle: Stairs (Robust)
    2013-05-21

    Digs a simple stair and places missing ground blocks and torches (every 4th horizontal block).

    Usage:
    - use turtle and type "label set <name>"
      (to give your turtle an unique name so it remembers its programs)
    - type "pastebin get hT1hG700 stairs"
    - place solid blocks in slot 16 (lower right corner)
    - place torches in slot 15 (left of slot 16)
    - type "stairs <height> [<limit>]"
        • where <height> represents the height of the hallway (3-5 seems good)
        • and <limit> (optional) represents the Y coordinates digged down
          (if you're at y64 and type in 10, it will dig until it's at y54)
--]]

local args = { ... }
if #args < 1 then
  print( "Usage: stairs <height> [<limit>]" )
  error()
end
height = tonumber(args[1])
limit = 256
if #args == 2 then
    limit = tonumber(args[2])
end

if height < 2 then
  print( "The height must be >= 2." )
  error()
end
    print("======== 2013 (c) psychedelixx ========")
    print("Let's go!")

    while limit > 0 do

            if turtle.getFuelLevel() < height*2 then
                    turtle.refuel()
            end
            if turtle.getFuelLevel() < height*2 then
                    print("Fuel is almost empty!")
            end

        print("")
        print("Remaining: " .. limit)
        print("Next torch in: " .. limit%4 .. "m")

        for var = 0, height-3 do
            t.digUp()
            if var < height-3 then
                t.up()
            end
            if var == 0 and limit%4 == 0 then
                t.right()
                t.dig()
                turtle.select(15)
                turtle.place()
                t.left()
            end
        end
        for var = 0, height-4 do turtle.down() end
        t.digDown()
        t.down()
        if not turtle.detectDown() then
            turtle.select(16)
            turtle.placeDown()
        end

            t.dig()

        turtle.forward()
        limit = limit-1
    end