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From The Free On-line Dictionary of Computing (27 SEP 03) : [ foldoc ]
negation by failure
An extralogical feature of Prolog and other logic
programming languages in which failure of unification is
treated as establishing the negation of a relation. For
example, if Ronald Reagan is not in our database and we
asked if he was an American, Prolog would answer "no".
(1994-11-29)