Choose {TAIL ~ a value} of {HEAD ~ z} and run through the k values in: d=z+k 
Choose { TAIL ~ a value } of { HEAD ~ z } and run through the k values in : d = z+k
OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH