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