I tried running your code using commercial Macsyma (changed pdiff to
ndiff). Macsyma halts with an error (something about map).
Your code is difficult for me to understand (ev, double quote, single
quotes,...), but I'd guess there is a much easier way to do the
calculation.
The pdiff code is ugly. It clobbers several Maxima functions.
File a bug report---things on the list are sometimes forgotten.
Barton
author of pdiff