Built in Predicates

Rules may include atoms with built-in predicates:

ExpensiveProduct(X) :- Product(X,Y,P) & P > $100

But: we need to restrict the use of built-in atoms in rules.

P(X) :- R(X) & X<Y

What does this mean?

Hence, we require that every variable that appears in a built-in

atom also appears in a relational atom.

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