How to specify and verify Horn clauses (logic programming programs)? Semantics of Horn clauses
You should have a look at Uniform Proofs as a Foudation for Logic Programming by Dale Miller, Gopalan Nadathur, Frank Pfenning and Andre Scedrov, 1991. The idea of this work and the rich area that has seen spawn from these ideas is to understand the execution of a logic program as proof search in simple and expressive logics, such as intuitionistic logic or linear logic. A particular class of formulas with good proof search properties are the Hereditary Harrop Formulas, which nicely generalize Horn Clauses.