The DLV system is a disjunctive logic programming system, implementing the stable model semantics under the Answer set programming paradigm. It extends the datalog language to allow the use of OR in rules. The tutorial page gives a simple example modeling some laughing when told a joke.
joke. laugh :- joke.
The following would represent the case where a joke was not told.
laugh :- joke.
Since the value of joke is not indicated as true in the second program it is assumed to be false.