ComputerCraft Turtle StripMine

function Shaft()
for a = 1,57 do
for b= 1,4 do
turtle.dig()
turtle.turnLeft()
end
turtle.digDown()
turtle.down()
end
end

function Return()
for a=1,57 do
turtle.up()
end
end

function Movement()
a=0
for b=1,2 do
turtle.forward()
end
a=2+a
if a>=4 then
turtle.turnLeft()
turtle.turnLeft()
for c=1,a do
turtle.forward()
end
for d=1,16 do
turtle.select(d)
turtle.drop()
end
turtle.turnLeft()
turtle.turnLeft()
for d=1,a do
turtle.forward()
end
end
end

for z=1,10 do
Movement()
Shaft()
Return()
end