Negated Subgoals

Rules may include negated subgoals, but in restricted forms:

Ok:

P(X,Y) :- Between(X,Y,Z) & NOT Direct(X,Z)

Bad:

Q(X, Y) :- R(X) & NOT S(Y)

Bad but salvagable:

T(X) :- R(X) & NOT S(X,Y)

We’ll rewrite as:

S’(X) :- S(X,Y)

T(X) :- R(X) & NOT S’(X)