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# Rules

The format is:

```
divisible_by_two:-

even.

```
This is a non-unit clause.

In general, a clause consists of two parts: the head and the body.

The head is divisible_by_two and the body is even --- even is sometimes referred to as a subgoal.

Note that the symbol ``:-'' is read as if. An informal reading of the clause is `` divisible_by_two is true if even is true'' which is equivalent to `` even divisible_by_two''.

Any number of subgoals may be in the body of the rule.

This is another way in which Prolog is a restriction of full first order predicate calculus. For example, we cannot translate rich(fred) happy(fred)powerful(fred) directly into the Prolog version happy(fred),powerful(fred) :- rich(fred).

See section 2.10 for an example of a clause with more than one subgoal in the body. A fact is effectively a rule with no subgoals.

You may have noticed that, even if it is held that ``even'' is a relation, it does not seem to relate anything to anything else.

The rule is not as much use as it might be because it does not reveal the more interesting relationship that

A number is divisible by two if it is even
We can express this with the help of the logical variable. Here is the improved rule:
```
divisible_by_two(X):-

even(X).

```
This is also a non-unit clause. The named logical variable is X. This Prolog clause is equivalent to the predicate calculus statement X. (even(X) divisible_by_two(X)).

Paul Brna
Mon May 24 20:14:48 BST 1999