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