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In intuitionistic logic, the Harrop formulae, named after Ronald Harrop, are the class of formulae inductively defined as follows: * Atomic formulae are Harrop, including falsity (⊥); * is Harrop provided and are; * is Harrop for any well-formed formula ; * is Harrop provided is, and is any well-formed formula; * is Harrop provided is. By excluding disjunction and existential quantification (except in the antecedent of implication), non-constructive predicates are avoided, which has benefits for computer implementation. From a constructivist point of view, Harrop formulae are "well-behaved." For example, in Heyting arithmetic, Harrop formulae satisfy a classical equivalence not usually satisfied in constructive logic:〔 : Harrop formulae were introduced around 1956 by Ronald Harrop and independently by Helena Rasiowa.〔 Variations of the fundamental concept are used in different branches of constructive mathematics and logic programming. ==Hereditary Harrop formulae and logic programming== A more complex definition of hereditary Harrop formulae is used in logic programming as a generalisation of horn clauses, and forms the basis for the language λProlog. Hereditary Harrop formulae are defined in terms of two (sometimes three) recursive sets of formulae. In one formulation:〔Dov M. Gabbay, Christopher John Hogger, John Alan Robinson, ''Handbook of Logic in Artificial Intelligence and Logic Programming: Logic programming'', Oxford University Press, 1998, p 575, ISBN 0-19-853792-1〕 * Rigid atomic formulae, i.e. constants or formulae , are hereditary Harrop; * is hereditary Harrop provided and are; * is hereditary Harrop provided is; * is hereditary Harrop provided is rigidly atomic, and is a ''G''-formula. ''G''-formulae are defined as follows:〔 * Atomic formulae are ''G''-formulae, including truth(⊤); * is a ''G''-formula provided and are; * is a ''G''-formula provided and are; * is a ''G''-formula provided is; * is a ''G''-formula provided is; * is a ''G''-formula provided is, and is hereditary Harrop. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Harrop formula」の詳細全文を読む スポンサード リンク
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