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The well-founded model provides a natural and robust semantics
for logic programs with negative literals in rule bodies. We implemented
the well-founded semantics in the SLG-WAM
Performance results indicate that the overhead of delay and
simplification to Prolog -- or tabled -- evaluations is minimal.
To compute the well-founded semantics, the SLG-WAM adds to an efficient
tabling engine for definite programs three operations -- negative loop
detection, delay and simplification -- which serve to detect, to break
and to resolve cycles through negation that might arise in evaluating
XSB is a full Prolog system that closely approximates the ISO standard;
additionally, it supports a tight integration of tabled predicates
with nontabled predicates.