{HEAD ~ The system} 7, wherein {TAIL ~ the first and second diffractive structures} are on a same surface 
wherein
{ HEAD ~ The system } 7 , wherein { TAIL ~ the first and second diffractive structures } are on a same surface
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH