2high tunnel with floor

-- 2high
targs = { ... }

length = tonumber(targs[1])

for i = 1,length do
    if not turtle.detectDown() then
        turtle.placeDown()
    end
    if turtle.detectUp() then
        turtle.digUp()
    end
    if turtle.detect() then
        turtle.dig()
    end
    if i<length then
        turtle.forward()
    end
end