  Forsaking the traditionnal hand-waving in the treatment of the motion, we
show that the ultra-relativistic approximation and the equality of kinematical
variables are unnecessary ingredients in the derivation of the oscillation
length using plane waves, at least in a two flavor world. It ensues that the
formula is valid as it is in the non relativistic regime, provided one uses the
correct variable which is found to be momentum, not energy, and that the
precise production kinematics is irrelevant. Consequences for the more complete
treatments are briefly evoked.
