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