The first reflective lateral face of the first triangular-pyramidal retroreflective
unit is on the same plane with the first lateral face of the tetrahedral retroreflective
unit, the second reflective lateral face of the first triangular-pyramidal retroreflective
unit is on the same plane with the second lateral face of the tetrahedral retroreflective
unit, the third reflective lateral face of the first triangular-pyramidal retroreflective
unit is parallel to one of the two lateral faces forming a V-shaped groove, the
third reflective lateral face of the second triangular-pyramidal retroreflective
unit is identical with, or parallel to, the other of the two faces forming said
V-shaped groove, and the third reflective lateral face of said tetrahedral retroreflective
unit is same as one of the two faces forming said V-shaped groove.
