Computercraft Turtle Tree Chopping

-- Tree farm program
-- Place fir, spruce, or birch saplings in slot 1
-- (will work with oak, but if it grows a big oak tree the program will miss the branches)
-- place bonemeal in slot 2
-- call the program by typing the program name followed by the number of trees you want chopped down
-- check me out at Http://www.youtube.com/chronnotrigg

local trees = {...}

function plantTree()
        turtle.select(1)
        turtle.place()
        turtle.select(2)
        turtle.place()
end

function chopTree()
        local p = 0
    for i=0,128 do
        turtle.select(3)
        if turtle.compareUp() == true then
            turtle.digUp()
            turtle.up()
            p = p + 1
        else
            break
        end
    end
    for i=1,p do
        turtle.down()
    end
end

    -- main
if trees[1] == 0 then
        print("argument must be 1 or larger")
        return
elseif tonumber(trees[1]) == nil then
        trees = 1
end
for i=1,trees[1] do
        plantTree()
        turtle.dig()
        turtle.forward()
        chopTree()
        turtle.back()
end