For example, referring to FIG 3, d1, separation between {TAIL ~ rows} 320 and 321; d2, separation between rows 321 and 322; and d3, separation between {HEAD ~ rows} 322 and 323; need not be equal, although in some embodiments of the invention, d1, d2 and d3 may be substantially equal 
For example , referring to FIG 3 , d1 , separation between { TAIL ~ rows } 320 and 321 ; d2 , separation between rows 321 and 322 ; and d3 , separation between { HEAD ~ rows } 322 and 323 ; need not be equal , although in some embodiments of the invention , d1 , d2 and d3 may be substantially equal
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH