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Recent developments in higher-order logics and theorem prover design have led to an

We set up a formal framework to describe term deduction systems, such as transition system speci cations in the style of Plotkin, and conditional term rewriting systems. This framework has the power to express many-sortedness, general binding mechanisms and substitutions, among other notions such as negative premises and unary predicates on terms. The framework is used to present a conservativity format in operational semantics, which states su cient criteria to ensure that the extension of a transition system speci cation with new rules does not a ect the behaviour of the original terms. Furthermore, we showhowgeneral theorems in structured operational semantics can be transformed into results in conditional term rewriting. We apply this approach to the conservativity theorem, which yields a result that is useful in the eld of abstract data types. 1

Theorem provers for higher-order logics often use tactics to implement automated proof search. Tactics use a general-purpose meta-language to implement both general-purpose reasoning and computationally intensive domain-specific proof procedures. The generality of tactic provers has a performance penalty; the speed of proof search lags far behind special-purpose provers. We present a new modular proving architecture that significantly increases the speed of the core logic engine.

The topic of this thesis is the extraction of efficient and readable programs from formal constructive proofs of decidability. The proof methods employed to generate the efficient code are new and result in clean and readable Nuprl extracts for two non-trivial programs. They are based on the use of Nuprl's set type and techniques for extracting efficient programs from induction principles. The constructive formal theories required to express the decidability theorems are of independent interest. They formally circumscribe the mathematical knowledge needed to understand the derived algorithms. The formal theories express concepts that are taught at the senior college level. The decidability proofs themselves, depending on this material, are of interest and are presented in some detail. The proof of decidability of classical propositional logic is relative to a semantics based on Kleene's strong three-valued logic. The constructive proof of intuitionistic decidability presented here is the first machine formalization of this proof. The exposition reveals aspects of the Nuprl tactic collection relevant to the creation of readable proofs; clear extracts and efficient code are illustrated in the discussion of the proofs.

. This paper presents a formalization of a sequent presentation of intuitionisitic propositional logic and proof of decidability.The proof is implemented in the Nuprl system and the resulting proof object yields a "correct-by-construction" program for deciding intuitionisitc propositional sequents. The extracted program turns out to be an implementation of the tableau algorithm. If the argument to the resulting decision procedure is a valid sequent, a formal proof of that fact is returned, otherwise a counter-example in the form of a Kripke Countermodel is returned. The formalization roughly follows Aitken, Constable and Underwood's presentation in [1] but a number of adjustments and corrections have been made to ensure the extracted program is clean( no non-computational junk) and efficient. 1