conditional literals - clingo 5 vs clingo 4

[wdvorak]

We are using several asp encodings that make use of conditional literals and noticed that on some instances clingo 4 (4.4.0) and clingo 5 (5.4.0) return different answer-sets. It seems that this only happens for programs having conditional literals in rule heads.

For instance for the program below

arg(a1). arg(a2). arg(a3). arg(a4). arg(a5). arg(a6). arg(a7). arg(a8). arg(a9).
att(a2,a1). att(a2,a6). att(a2,a7). att(a2,a8). att(a2,a9).
att(a3,a1).
att(a4,a1).
att(a5,a1).
att(a6,a1). att(a6,a2). att(a6,a3). att(a6,a4). att(a6,a5).
att(a7,a1).
att(a8,a1).
att(a9,a1).
in(X) :- arg(X), not out(X).
out(X) :- arg(X), not in(X).
in(X):-arg(X), out(Y):att(Y,X).
out(X):- in(Y), att(X,Y).
out(X):-in(Y), att(Y,X).
in(Y):att(Y,X) :-out(X).
#show in/1.
#show out/1.
:-in(a1).

clingo4 returns the answer-sets

Answer: 1
out(a1) out(a2) out(a3) out(a4) out(a5) in(a6) in(a7) in(a8) in(a9)
Answer: 2
out(a1) in(a2) in(a3) in(a4) in(a5) out(a6) out(a7) out(a8) out(a9)

while clingo5 returns UNSATISFIABLE.
When removing the last constraint ':-in(a1).' then both versions return the two answer-sets listed above.

Is there an intended difference in the semantics of conditional literals between clingo 4 and clingo 5?

Another example where clingo 4 and clingo 5 disagree on the answer-sets is
idealset_adm.txt

[rkaminsk]

@BenKaufmann: This seems to be a regression in clasp.

The program bug.sm.gz grounded with

$ clingo --mode=gringo bug.lp | lpconvert > bug.sm

produces different results depending on the clasp version used. Compare

$ clasp-3.1.5-mt bug.sm -V0
out(a1) out(a2) out(a3) out(a4) out(a5) in(a6) in(a7) in(a8) in(a9)
SATISFIABLE

and

clasp-3.2.0-mt bug.sm -V0 
*** ERROR: (clasp): shifting must not introduce new atoms!
UNKNOWN

and

$ clasp-3.3.5-mt bug.sm -V0
UNSATISFIABLE

[BenKaufmann]

@rkaminsk
This is indeed a regression in clasp. Should be fixed in dev (potassco/clasp@9389f60).

[rkaminsk]

Thanks @BenKaufmann. There are still some other things in clingo I want to address before doing a release. @wdvorak hope you can live an older clingo version for now or apply the patch yourself.

[wdvorak]

thanks for your quick response. We are fine with using the older version for now and are looking forward to the next release ;).

[rkaminsk]

Fixed in wip branch updating clasp with 4db3c47.