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