In this review paper a development stages of logic programming theory and practice of usage
are analyzed. Special attention is paid to the Horn clause, which formed the basis of computer programs
written in logic. The syntax and step by step transformation analysis is described, starting from atomic
formula of predicate logic to the logical model of the real world, presented in Conjunctively-normal form
and consisting of Horn clause set only. This set is almost the real program code written in the Prolog - the
logic programming language. It is mentioned that the resulting form is interpreted by the inference engine,
which is for example, the interpreter of Prolog programming language.
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