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