  We propose a simple set of hypotheses governing the deviations of the
leptonic mapping matrix from the Harrison-Perkins-Scott (HPS) form. These
deviations are supposed to arise entirely from a perturbation of the mass
matrix in the charged lepton sector. The perturbing matrix is assumed to be
purely imaginary (thus maximally $T$-violating) and to have a strength in
energy scale no greater (but perhaps smaller) than the muon mass. As we shall
show, it then follows that the absolute value of the mapping matrix elements
pertaining to the tau lepton deviate by no more than $O((m_\mu/m_\tau)^2)
\simeq 3.5 \times 10^{-3}$ from their HPS values.
  Assuming that $(m_\mu/m_\tau)^2 $ can be neglected, we derive two simple
constraints on the four parameters $\theta_{12}$, $\theta_{23}$, $\theta_{31}$,
and $\delta$ of the mapping matrix. These constraints are independent of the
details of the imaginary $T$-violating perturbation of the charged lepton mass
matrix. We also show that the $e$ and $\mu$ parts of the mapping matrix have a
definite form governed by two parameters $\alpha$ and $\beta$; any deviation of
order $m_\mu/m_\tau $ can be accommodated by adjusting these two parameters.
