  Criticality in graviational microlensing is an everyday issue because that is
what generates microlensing signals which may be of photon-challenged compact
objects such as black holes or planetary systems ET calls home. The criticality
of these quasi-analytic lenses is intrinsically quadratic, and the critical
curve behaves as a mirror generating two mirror images along the image line
(parallel +/- E_-) at the same distances from the critical curve in the
opposite sides.
  At the (pre)cusps where the caustic curve "reflects" and develops cusps,
however, "would-be" two pairs of quadratic images "superpose" to produce three
mirror images because of the degenerate criticality. The critical curve behaves
as a parabolic mirror, and the image inside the parabola is indeed a superposed
image having the sum of the magnifications of the other two that are outside
the parabola. All three images lie on a parabolic image curve shaped by a
property (nabla J) of the (pre)cusp, and two distances of the system determine
the position of the image curve and positions of the three images on the image
curve. The triplet images satisfy sum J^{-1} = 0, and the function sum J^{-1}
is discontinuous at a caustic crossing where a pair of quadratic images
disappear into a critical point. The reflection symmetry of the image curve is
a manifestation of the symmetry of the cusp which is also respected by a trio
of parabolic curves that are tangnet at the (pre)cusp and define the image
domains. The symmetry is guaranteed when J_{+-} vanishes or can be ignored, and
the cusps on the lens axis of the binary lenses are strongly symmetric having
J_{+-} = 0 because of the global reflection symmetry of the binary lenses.
"E_{+/-}-algebra" is laid out for users' convenience.
