  We give a convenient expression for the appearance probability P(nu_mu ->
nu_e) describing neutrino oscillations in matter of constant density, derived
using textbook quantum mechanics stratagems. Our formulation retains the
clarity of an expansion in alpha = Delta m_{21}^2/Delta m_{31}^2 exhibited by
the popular Cervera et al. formula [Nucl. Phys. B 579, 17 (2000)] while
enabling more accurate evaluation of oscillations over terrestrial baselines.
