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