Horn clauses express a subset of statements of first-order logic. Programming language Prolog is built on top of Horn clauses. Prolog programs
are comprised of definite clauses and any question
in Prolog is a goal.
Clocksin, W. F. and Mellish, C. S. Programming in Prolog Using the ISO Standard. New York: Springer-Verlag, 1984.Horn,
A. "On Sentences Which Are True of Direct Unions of Algebras." J. Symbolic
Logic16, 14-21, 1951.