a tightly folded zig zag optical path within said block between said pair of {HEAD ~ opposed sides} having {TAIL ~ vertices} at a preselected fold angle, 
a tightly folded zig zag optical path within said block between said pair of { HEAD ~ opposed sides } having { TAIL ~ vertices } at a preselected fold angle ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH