computer-craft/lumberjack

function up()
  turtle.digUp()
  turtle.up()
end

function get_down()
  while turtle.down() do end
end

function dig()
  return turtle.dig()
end


function cut_tree()
  success = dig()
  if success then
    up()
    cut_tree()
  else
    get_down()
  end
end

cut_tree()