Horn clause

logic A set of atomic literals with at most one positiveliteral. Usually written

L #@- L1, ..., Ln

or

- L1, ..., Ln

where n=0. If L is false the clause is regarded as a goal.

Horn clauses can express a subset of statements of firstorder logic.

The name "Horn Clause" comes from the logician Alfred Horn,

who first pointed out the significance of such clauses in

1951, in the article "On sentences which are true of direct

unions of algebras", Journal of Symbolic Logic, 16, 14-21.

A definite clause is a Horn clause that has exactly one

positive literal.

(2000-01-24)