Semantics

Here is an informal version of the procedural semantics for the example above:

If we can find a value of X that satisfies the goal even(X) then we have also found a number that satisfies the goal divisible_by_two(X).

If we can prove that X is ``even'' then we have proved that X is ``divisible_by_two''.

Note that there is an implicit universal quantification here. That is, for all objects those that are even are also divisible by two.

X.(even(X) divisible_by_two(X))

Also note that the head goal is found on the right of the standard logical implication symbol. It is a common error to reverse the implication.

Two final examples of a single rule. The first:

all scots people are british

 
british(Person):-

		 scottish(Person).

if you go from one country to another they you are a tourist

 
tourist(P):-

		 move(P,Country1,Country2).

There is a problem here. We really need to specify that Country1 and Country2 are legitimate and distinct countries.