In {TAIL ~ alternate embodiments}, {HEAD ~ stacking arrangements} include: gamma structures alone; or delta structures alone 
In { TAIL ~ alternate embodiments } , { HEAD ~ stacking arrangements } include : gamma structures alone ; or delta structures alone
REL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH