Question about partial-order answer subsumption

Help!
1

Hi !
I would like to set up tabling so that only the most specific calls are tabled. So I have written the following code:

:- table p(_,po(isSubsumedBy/2),_).

% I tested also the following directive to compare with the one above
% :- table p/3.

isSubsumedBy(T1,T2):- subsumes_term(T2,T1).

p(1,_,3).
p(1,2,3).

In order to see the content of the table, I’ve written the following predicates (display_table displays all memoized calls with the list of their associated keys):

display_table:-
    findall([T,LKey],
            (current_table(T,Trie),
             findall(Key,(trie_gen_compiled(Trie, Key)), LKey)),
            Res),
    writeln("Current table :"),
    writeList(Res).

writeList(L):-
	write("["),
	writeList2(L),
	writeln("]").
writeList2([]).
writeList2([H]):-
	write(H).
writeList2([H|[H2|T]]) :-
	writeln(H), 
	writeList2([H2|T]).

Then I obtain the following results. First I give the reference tests (not using partial-order answer subsumption during tabling):

/*
SWI-Prolog threaded, 64 bits, version 8.4.2
With :- table p/3.
we get: 
*/

2 ?- p(X,Z,Y).
X = 1,
Y = 3 ;
X = 1,
Z = 2,
Y = 3.
      
3 ?- display_table.
Current table :
[[p(_14378,_14380,_14382),[ret(1,_14400,3),ret(1,2,3)]]]
true ;
false.
      
4 ?-  p(1,2,3).
true.

That’s ok, everything works fine.
Now I want to keep only the most specific call in the table, ie p(1,2,3), discarding p(1,_,3). I obtain with SWI 8.4.2:

/*
SWI-Prolog threaded, 64 bits, version 8.4.2
With :- table p(_,po(isSubsumedBy/2),_).
we get:
*/

2 ?- p(X,Z,Y).
X = 1,
Z = 2,
Y = 3.

Up to here, that’s exactly what I want, but problems occur with subsequent queries:

      
3 ?- display_table.
Current table :
[[p(_12788,_12790,_12792),[]]]
true ;
false.

This does not seem coherent with the first query.


4 ?- p(1,2,3).
ERROR: Uninstantiated argument expected, found 2
ERROR: In:
ERROR:   [12] throw(error(uninstantiation_error(2),_23840))
ERROR:   [10] '$wrap$p'(1,2,3)1-st clause of '$wrap$p'/3 <no source>
ERROR:    [9] toplevel_call(user:user: ...) at c:/program files/swipl842/boot/toplevel.pl:1158
ERROR: 
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.

I do not understand what happens here.
I also made the same tests with SWI 8.5.17:

/*
SWI-Prolog threaded, 64 bits, version 8.5.17
With :- table p(_,po(isSubsumedBy/2),_).
we get:
*/

2 ?- p(X,Z,Y).
X = 1,
Z = 2,
Y = 3.
      
3 ?- display_table.
Assertion failed: isAttVar(*av), file /home/swipl/src/swipl-devel/src/pl-attvar.c, line 173

Once again I do not understand what happens. It is likely I have not fully understood partial-order answer subsumption, but I cannot say.
Could you help me ?

Thanks