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