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