## Among the following which is not a horn clause?

**...**

**Horn Clause**

The definite clause language does not allow a contradiction to be stated. However, a simple expansion of the language can allow proof by contradiction.

An integrity constraint is a clause of the form

*false←a*

_{1}∧...∧a_{k}.

where the a

_{i}are atoms and false is a special atom that is false in all interpretations.

*A*

**is either a definite clause or an integrity constraint. That is, a Horn clause has either false or a normal atom as its head.**

*Horn clause*Integrity constraints allow the system to prove that some conjunction of atoms is

*false*in all models of a knowledge base - that is, to prove disjunctions of negations of atoms. Recall that*¬p*is the negation of*p*, which is true in an interpretation when*p*is false in that interpretation, and*p∨q*is the disjunction of*p*and*q*, which is true in an interpretation if*p*is true or*q*is true or both are true in the interpretation. The integrity constraint*false←a*is logically equivalent to_{1}∧...∧a_{k}*¬a*._{1}∨...∨¬a_{k}Your can read more here with examples.

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