  Presymmetry, the hidden symmetry underlying the charge and generational
patterns of quarks and leptons, is utilized for repairing the left-right
asymmetry of the standard model with Dirac neutrinos. It is shown that the
restoration of parity is consequent with an indispensable left-right symmetric
residual presymmetry. Thus, presymmetry substantiates left-right symmetry and
the experimental search for the latter is the test of the former, with the
nature of neutrinos as a crucial feature that can distinguish the left-right
symmetry alone and its combination with presymmetry. This phenomenological
implication is in accordance with the fact that Majorana neutrinos are usually
demanded in the first case, but forbidden in the second.
