Resolution

• Ordering is especially important for recursive rule sets

• The two sets of rules on the right are logically equivalent

  • Conjunction is commutative

• But they are computationally very different

ancestor(X,Y) :- parent(X,Y).

ancestor(X,Y) :-

parent(X,Z),

ancestor(Z,Y).

ancestor(X,Y) :- parent(X,Y).

ancestor(X,Y) :-

ancestor(X,Z),

parent(Z,Y).

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