MyComputerCraft

local TreeHeight = 7

function Mine(height)
	turtle.dig()
	turtle.forward()

	for i=1,height do
		turtle.digUp()
		turtle.up()
	end

	for i=1,height do
		turtle.down()
	end
end

Mine(TreeHeight)
Mine(TreeHeight)
turtle.turnRight()
Mine(TreeHeight)
turtle.turnRight()
Mine(TreeHeight)