{TAIL ~ a circuit} formed of {HEAD ~ first, second, third and fourth transistors} whose sources or emitters are coupled together 
{ TAIL ~ a circuit } formed of { HEAD ~ first , second , third and fourth transistors } whose sources or emitters are coupled together
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH