Subtleties with Stratified Rules

Example:

p(X) :- r(X)

q(X) :- s(X) & NOT p(X).

Suppose: R = {1}, and S = {1,2}

One solution: P = {1} and Q = {2}

Another solution: P={1,2} and Q={}.

Perfect model semantics: apply the rules stratum after stratum.

Previous slide

Back to first slide

View graphic version