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### 13.6. Rules, Axioms, and Programs

A rule is a series of structures in which the truth of the first structure follows from the ability to prove the remaining structures. For example, in Logikus the following rule defines “high” cities:

```highCity(Name, Alt) :- city(Name, Alt), >(Alt, 5000);
```

This rule states that highCity(Name, Alt) is true for any known city whose altitude is greater than 5,000. A rule's first structure is its head; its other structure is its tail. Between its head and its tail a rule has the “if” (“:-) symbol. The head is true if the tail is provable.

An axiom is a model of truth, and in Logikus an axiom is either a fact or a rule. A Logikus program is a collection of axioms, and this means that a program is an extended model ...

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