mysterious "null node" warning had been printed out. It turned
out that this was being emitted by find_answers_for_subgoal(),
which is used in cascading abolishes. The problem arose for
deleting calls that had empty answer tries, where a stack was
checked, a null node immediately found, and the routine exited.
In this case, it was harmless, but I added a check to get rid of
the warning in this case, and made the warning itself more
descriptive.
