{HEAD ~ The fixed nonlinear function processing units} in a given stage implement only the nonlinear function needed for the MD5 round being performed by {TAIL ~ the stage} 
{ HEAD ~ The fixed nonlinear function processing units } in a given stage implement only the nonlinear function needed for the MD5 round being performed by { TAIL ~ the stage }
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL