Bug on XSB prolog???

Hi,

We have tried to use XBS to test one of our scenarios.
However, the result is not what we expect. The program
is trimed down as follow:

p :- sk_not(q).
q :- sk_not(p).

XSB is supposed to answer 'undefined' for both p and q
according to the WF semantics. But XSB answers 'yes'.
Have anyone found the same problem? or we have had
wrong configurations/versions? We are using XBS 2.7.1.

Regards,
Ha

.

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