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