There are several different possible wall configurations which include combinations of {HEAD ~ single walls} and {TAIL ~ double walls} 
There are several different possible wall configurations which include combinations of { HEAD ~ single walls } and { TAIL ~ double walls }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL