During an OCT propagation, the code may write the wavefunctions at some time steps (the
"check points"). When the inverse backward or forward propagation
is performed in a following step, the wavefunction should reverse its path
(almost) exactly. This can be checked to make sure that it is the case – otherwise
one should try reducing the time-step, or altering in some other way the
variables that control the propagation.
If the backward (or forward) propagation is not retracing the steps of the previous
forward (or backward) propagation, the code will write a warning.