{TAIL ~ The precursor} is relatively reactive under one set of conditions and is relatively non reactive under another set of {HEAD ~ conditions} 
{ TAIL ~ The precursor } is relatively reactive under one set of conditions and is relatively non reactive under another set of { HEAD ~ conditions }
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD