Therefore, even if a main course box is out of order {HEAD ~ the data} remains in {TAIL ~ the sub course box} 
remains in
Therefore , even if a main course box is out of order { HEAD ~ the data } remains in { TAIL ~ the sub course box }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL