fixceil

function goodblock()
  if not(turtle.detectUp()) then
    return false
  end
  for ii=1,4,1 do
    turtle.select(ii)
    if turtle.compareUp() then
      return true
    end
  end
  return false
end
function fixCeil()
  while turtle.detectUp() and not(goodblock()) do
    turtle.digUp()
    sleep(1)
  end
  while turtle.getItemCount(curslot)<2 do
    curslot=curslot+1
    if curslot>4 then
      curslot=1
    end
    turtle.select(curslot)
    sleep(.3)
  end
  turtle.placeUp()
end
curslot=1
while turtle.forward() do
  if not(goodblock()) then
    fixCeil()
  end
end