[Xsb-development] Abolishing tables with conditional answers

[Terrance S. <ts...@cs...>]

All:

With the help of a student, Dao Tran Minh, I'm exploring altering
the behavior of the table abolish routines when the tables
contain conditional answers.  This won't affect most users, but
will affect those who inspect the residual program (like the UNL
group) and possibly may affect some Flora users.

Recall that conditional answers contain numerous cross table
pointers in their delay lists and delay elements.  So if you have
a conditional answer

p(a):- q(a),r|

the conditional answer information for the answer p(a) contains a
pointer to the answer q(a) and the subgoal r, and each of these
elements contain backpointer to the answer p(a).  The forward
pointers are useful for introspecting the residual program, while
the back pointers are useful for simplification.

One issue we have is if abolish_table_subgoal(r) were called, the
pointer to r in the conditional answer above would be dangling,
and if get_residual(p(a),X) were later called, we'd get a core
dump.  This situation has happened in the past.

So a fix to this is, whenever we abolish a tabled subgoal (S) we
transitively abolish all subgoals containing an answer that
depends on S (or a transitively abolished subgoal) or an answer
to S (or a transitively abolished subgoal).  Also whenever we
abolish a tabled predicate, we abolish all tabled predicates just
as we did tabled subgoals above.  Of course, if the subgoal, or
predicate did not have a conditinoal answer, the behavior would
be unchanged.

My proposal is to represent the default behavior of
abolish_table_subgoal or abolish_table_pred (and perhaps some of
the other table abolishing routines) in a flag indicating whether
to abolish depending tables or not, and to make the default
behavior that of abolishing depending tables.  If needed, we
could also put options lists into the table abolishing routines.

Please let me know if you have any comments or suggestions.  If I
don't hear anything, I'll go ahead and make my changes.

Terry