Stratified Negation is Ok

A predicate P depends on a predicate Q if:

Q appears negated in a rule defining P.

If there is a cycle in the dependency graph, the datalog program

is not stratified.

Example:

p(X) :- r(X) & NOT q(X)

q(X) :- r(X) & NOT p(X)

Suppose r has the tuple {1}

What is the fixed point?