{HEAD ~ One reflector} in each module is concave, and the modules are asymmetric in section, although {TAIL ~ two asymmetric modules} may be combined in a system which is itself symmetrical 
{ HEAD ~ One reflector } in each module is concave , and the modules are asymmetric in section , although { TAIL ~ two asymmetric modules } may be combined in a system which is itself symmetrical
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH