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Failure

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The invocation of a predicate can fail for two reasons: the predicate is not applicable for the given input values, or the returned output values do not match the corresponding values in the invocation.

Consider


   IsJimsColor(yellow)

This invocation will fail because IsJimsColor is only applicable for red. An invocation fails if there is no applicable rule.

Consider also


   JimsColor(-> yellow)

This invocation will fail since JimsColor returns red which does not match yellow.

Failure does not mean error. As we shall see, it can be used to control predicate evaluation.

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