{TAIL ~ u} and {HEAD ~ v} at each occurrence are each independently 0, 1, 2, or 3, preferably 0 or 1 
{ TAIL ~ u } and { HEAD ~ v } at each occurrence are each independently 0 , 1 , 2 , or 3 , preferably 0 or 1
TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH