{TAIL ~ FRAG} is {HEAD ~ a shiftable detectable species} which provides a detectable species when released from the reducible compound, and 
{ TAIL ~ FRAG } is { HEAD ~ a shiftable detectable species } which provides a detectable species when released from the reducible compound , and
TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH