Prolog/Variables

A PROLOG variable can represent anything; a number, a name, a structure, an array, something as complicated as the known universe. A PROLOG program works by constraining the variables until eventually they have particular values; then telling you what the values are. A simple program might be

and when you run it, the result will be

The program might not go as far as to constrain the variables to have exact values, so you might get

where the '_' means that you have a variable remaining as part of the solution.

You can also get a 'Yes' result for more than one value of the variables; this is called 'nondeterminism', and is OK. If no values of the variables will make a solution, PROLOG will say 'No'.

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