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