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