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