Bibliography
- Alfred Horn (1951), "On sentences which are true of direct unions of algebras", Journal of Symbolic Logic, 16, 14–21.
- Dowling, W. and Gallier, J. (1984), "Linear-time algorithms for testing the satisfiability of propositional Horn formulae". Journal of Logic Programming, 3, 267-284.
- M. van Emden and R. Kowalski The semantics of predicate logic as a programming language. Journal of ACM, Vol. 23, 733-742.
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