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