CH═CH and C═CH ; or R18 together with an adjacent R16, or R18 together with {HEAD ~ an adjacent R17}, form {TAIL ~ a CH═CH} or a CH═C group 
CH ═ CH and C ═ CH ; or R18 together with an adjacent R16 , or R18 together with { HEAD ~ an adjacent R17 } , form { TAIL ~ a CH ═ CH } or a CH ═ C group
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH