In preferred embodiments, {HEAD ~ each} of x and {TAIL ~ y}, independently, is an integer between about 2 and 6 
of
In preferred embodiments , { HEAD ~ each } of x and { TAIL ~ y } , independently , is an integer between about 2 and 6
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH