If {HEAD ~ a change} to {TAIL ~ line switching data transfer} is to take place, then a line to another switch is switched through the exchange 4 and the data are transferred to this by line switching 
to
If { HEAD ~ a change } to { TAIL ~ line switching data transfer } is to take place , then a line to another switch is switched through the exchange 4 and the data are transferred to this by line switching
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL 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 OTH