# Question about assertz **URL:** \ **Category:** Predicate\ **Created:** [September 16, 2021, 2:14pm UTC](https://swi-prolog.discourse.group/t/question-about-assertz/4452 "2021-09-16T14:14:03Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Christophe\_Rey](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@Christophe\_Rey](https://swi-prolog.discourse.group/u/Christophe_Rey)\ **Post date:** [September 16, 2021, 2:14pm UTC](https://swi-prolog.discourse.group/t/question-about-assertz/4452/1 "2021-09-16T14:14:03Z")
Hi Jan, Hi all, I am using SWI version 8.3.29-29-g584873769. My code is the following ```prolog :- table q/1 as incremental. :- dynamic q2/1 as incremental. :- dynamic q3/1 as incremental. q3(1) :- assertz(q2(1)). q(X) :- q2(X). q(X) :- q3(X). ``` The following execution is ok for me : ```prolog ?- q2(1). false. ?- q3(1). true. ?- q2(1). true. ?- q(1). true. ``` But if I stop and relaunch SWI, then I do not understand the following error when directly executing query q(1). ```prolog ?- q(1). ERROR: [Thread pdt_console_client_0_Default Process] '$idg_changed'/1: No permission to update variant `user:q(1)' ERROR: [Thread pdt_console_client_0_Default Process] No permission to update variant `user:q(1)' ERROR: [Thread pdt_console_client_0_Default Process] In: ERROR: [Thread pdt_console_client_0_Default Process] [27] assertz(q2(1)) ERROR: [Thread pdt_console_client_0_Default Process] [24] call('$toplevel':(q/1)(1)) at c:/program files/swipl/boot/init.pl:484 ERROR: [Thread pdt_console_client_0_Default Process] [23] reset('$toplevel':call(...),_11818,_11820) at c:/program files/swipl/boot/init.pl:572 ERROR: [Thread pdt_console_client_0_Default Process] [22] delim(ret,'$toplevel':call(...),115413760,[]) at c:/program files/swipl/boot/tabling.pl:606 ERROR: [Thread pdt_console_client_0_Default Process] [21] activate(ret,'$toplevel':call(...),115413760) at c:/program files/swipl/boot/tabling.pl:584 ERROR: [Thread pdt_console_client_0_Default Process] [20] run_leader(ret,'$toplevel':call(...),fresh(118661792,115413760),_11938,_11940) at c:/program files/swipl/boot/tabling.pl:570 ERROR: [Thread pdt_console_client_0_Default Process] [19] setup_call_catcher_cleanup('$tabling':'$idg_set_current'(_11996,(0000000006E71FF0)),'$tabling':run_leader(ret,...,...,_12014,_12016),_11984,'$tabling':finished_leader(_12026,_12028,...,...)) at c:/program files/swipl/boot/init.pl:646 ERROR: [Thread pdt_console_client_0_Default Process] [18] create_table((0000000006E71FF0),fresh(118661792,115413760),ret,user:q(1),'$toplevel':call(...)) at c:/program files/swipl/boot/tabling.pl:384 ERROR: [Thread pdt_console_client_0_Default Process] [17] catch('$tabling':create_table((0000000006E71FF0),...,ret,...,...),deadlock,'$tabling':restart_tabling((q/1),...,...)) at c:/program files/swipl/boot/init.pl:546 ERROR: [Thread pdt_console_client_0_Default Process] [16] start_tabling_2((q/1),user:q(1),'$toplevel':call(...),(0000000006E71FF0),fresh(118661792,115413760),ret) at c:/program files/swipl/boot/tabling.pl:370 ERROR: [Thread pdt_console_client_0_Default Process] [14] '$wrap$q'(1)1-st clause of '$wrap$q'/1 ERROR: [Thread pdt_console_client_0_Default Process] [13] toplevel_call(user:user: ...) at c:/program files/swipl/boot/toplevel.pl:1117 ERROR: [Thread pdt_console_client_0_Default Process] ERROR: [Thread pdt_console_client_0_Default Process] Note: some frames are missing due to last-call optimization. ERROR: [Thread pdt_console_client_0_Default Process] Re-run your program in debug mode (:- debug.) to get more detail. ``` Do I miss something ? Cheers Christophe ---
**Author:** ![Christophe\_Rey](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@Christophe\_Rey](https://swi-prolog.discourse.group/u/Christophe_Rey)\ **Post date:** [September 16, 2021, 3:05pm UTC](https://swi-prolog.discourse.group/t/question-about-assertz/4452/2 "2021-09-16T15:05:32Z")
My previous post comes from the fact that I wonder if there is a way to distinguish a “positive” false obtained for a fact after a proof has been found that this fact is false, from a “negative” false for a fact inferred since nothing can be proven about this fact. For example with ```prolog p(2) :- false. ``` then we cannot distinguish the following answers : ```prolog ?- p(2). false. ?- p(3). false. ``` Indeed, p(2) is false because we have a proof it is so, while p(3) is false because we cannot prove anything about it. Is there a way to make any distinction ? Cheers. Christophe ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [September 17, 2021, 7:50am UTC](https://swi-prolog.discourse.group/t/question-about-assertz/4452/3 "2021-09-17T07:50:19Z")
> [@Christophe\_Rey](#): > > But if I stop and relaunch SWI I don’t really get this. After stop and relaunch you are back at square one, no? Why would this be different this time? The error comes from trying to modify a dynamic predicate that it involved in an incremental re-evaluation. In other words, if p depends on a dynamic predicate d than (re)evaluating p may not change d. > [@Christophe\_Rey](#): > > I wonder if there is a way to distinguish a “positive” false obtained for a fact after a proof has been found that this fact is false, from a “negative” false for a fact inferred since nothing can be proven about this fact. I doubt it. That is why ASP and its recently discussed cousin sCASP introduce explicit negation, so we can state `-p(2)` to indicate that `p(2)` is explicitly **known** to be false. In Prolog, tabled or not, we only have NAF, negation as failure. ---
**Author:** ![Christophe\_Rey](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@Christophe\_Rey](https://swi-prolog.discourse.group/u/Christophe_Rey)\ **Post date:** [September 18, 2021, 11:50am UTC](https://swi-prolog.discourse.group/t/question-about-assertz/4452/4 "2021-09-18T11:50:17Z")
> [@jan](#): > > In other words, if p depends on a dynamic predicate d than (re)evaluating p may not change d. OK, thank you Jan, I think I get my mistake. > [@jan](#): > > That is why ASP and its recently discussed cousin sCASP introduce explicit negation, so we can state `-p(2)` to indicate that `p(2)` is explicitly **known** to be false. In Prolog, tabled or not, we only have NAF, negation as failure. Ok. So what does it mean when you implement sCASP into SWI ? Do you implement it by modifying core SWI mechanisms ? Or do you implement it by defining sCASP as an extra layer on top of NAF ? Sorry if this is not clear, and thank you for your answers. ---
**Author:** ![swi](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@swi](https://swi-prolog.discourse.group/u/swi)\ **Post date:** [September 18, 2021, 12:43pm UTC](https://swi-prolog.discourse.group/t/question-about-assertz/4452/5 "2021-09-18T12:43:11Z")
> [@Christophe\_Rey](#): > > Ok. So what does it mean when you implement sCASP into SWI ? Do you implement it by modifying core SWI mechanisms ? Or do you implement it by defining sCASP as an extra layer on top of NAF ? > > Sorry if this is not clear, and thank you for your answers. Christophe, sCASP is a system that was written using ciao prolog, and Jan is porting it to SWI Prolog making some enhancements along the way. You can already play with it (it is **not ready for normal use** ): [GitHub - JanWielemaker/sCASP: top-down interpreter for ASP programs with constraints](https://github.com/JanWielemaker/sCASP) ---
**Author:** ![Christophe\_Rey](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@Christophe\_Rey](https://swi-prolog.discourse.group/u/Christophe_Rey)\ **Post date:** [September 20, 2021, 7:11am UTC](https://swi-prolog.discourse.group/t/question-about-assertz/4452/6 "2021-09-20T07:11:59Z")
Thanks a lot, I will have a look at the link.