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A New Correctness Proof of the NelsonOppen Combination Procedure
 Frontiers of Combining Systems, volume 3 of Applied Logic Series
, 1996
"... The NelsonOppen combination procedure, which combines satisfiability procedures for a class of firstorder theories by propagation of equalities between variables, is one of the most general combination methods in the field of theory combination. We describe a new nondeterministic version of the p ..."
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Cited by 76 (4 self)
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The NelsonOppen combination procedure, which combines satisfiability procedures for a class of firstorder theories by propagation of equalities between variables, is one of the most general combination methods in the field of theory combination. We describe a new nondeterministic version of the procedure that has been used to extend the Constraint Logic Programming Scheme to unions of constraint theories. The correctness proof of the procedure that we give in this paper not only constitutes a novel and easier proof of Nelson and Oppen's original results, but also shows that equality sharing between the satisfiability procedures of the component theories, the main idea of the method, can be confined to a restricted set of variables.
Combining Symbolic Constraint Solvers on Algebraic Domains
 Journal of Symbolic Computation
, 1994
"... ion An atomic constraint p ? (t 1 ; : : : ; t m ) is decomposed into a conjunction of pure atomic constraints by introducing new equations of the form (x = ? t), where t is an alien subterm in the constraint and x is a variable that does not appear in p ? (t 1 ; : : : ; t m ). This is formalized tha ..."
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Cited by 28 (7 self)
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ion An atomic constraint p ? (t 1 ; : : : ; t m ) is decomposed into a conjunction of pure atomic constraints by introducing new equations of the form (x = ? t), where t is an alien subterm in the constraint and x is a variable that does not appear in p ? (t 1 ; : : : ; t m ). This is formalized thanks to the notion of abstraction. Definition 4.2. Let T be a set of terms such that 8t 2 T ; 8u 2 X [ SC; t 6= E1[E2 u: A variable abstraction of the set of terms T is a surjective mapping \Pi from T to a set of variables included in X such that 8s; t 2 T ; \Pi(s) = \Pi(t) if and only if s =E1[E2 t: \Pi \Gamma1 denotes any substitution (with possibly infinite domain) such that \Pi(\Pi \Gamma1 (x)) = x for any variable x in the range of \Pi. It is important to note that building a variable abstraction relies on the decidability of E 1 [ E 2 equality in order to abstract equal alien subterms by the same variable. Let T = fu #R j u 2 T (F [ X ) and u #R2 T (F [ X )n(X [ SC)g...
Combination Techniques for NonDisjoint Equational Theories
 Proceedings 12th International Conference on Automated Deduction
, 1994
"... ion variables which are variables coming from an abstraction, either during preprocessing or during the algorithm itself. 3. Introduced variables which are variables introduced by the unification algorithms for each theory. We make the very natural assumption that the unification algorithm for each ..."
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Cited by 25 (5 self)
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ion variables which are variables coming from an abstraction, either during preprocessing or during the algorithm itself. 3. Introduced variables which are variables introduced by the unification algorithms for each theory. We make the very natural assumption that the unification algorithm for each theory may recognize initial, abstraction and introduced variables and never assigns an introduced variable to a nonintroduced one or an abstraction variable to an initial one. With this assumption, our combination algorithm will always make an introduced variable appear in at most one \Gamma i . We may thus also suppose that the domain of each solution does not contain an introduced variable. This does not compromise the soundness of our algorithm. The combination algorithm is described by the two rules given in figure 2. In the rule UnifSolve i , ae SF is obtained by abstracting aliens in the range of ae by fresh variables. ae F i is the substitution such that xae = xae SF ae F i for al...
Cooperation of Decision Procedures for the Satisfiability Problem
 Frontiers of Combining Systems: Proceedings of the 1st International Workshop, Munich (Germany), Applied Logic
, 1996
"... : Constraint programming is strongly based on the use of solvers which are able to check satisfiability of constraints. We show in this paper a rulebased algorithm for solving in a modular way the satisfiability problem w.r.t. a class of theories Th. The case where Th is the union of two disjoint t ..."
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Cited by 22 (4 self)
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: Constraint programming is strongly based on the use of solvers which are able to check satisfiability of constraints. We show in this paper a rulebased algorithm for solving in a modular way the satisfiability problem w.r.t. a class of theories Th. The case where Th is the union of two disjoint theories Th 1 and Th 2 is known for a long time but we study here different cases where function symbols are shared by Th 1 and Th 2 . The chosen approach leads to a highly nondeterministic decomposition algorithm but drastically simplifies the understanding of the combination problem. The obtained decomposition algorithm is illustrated by the combination of nondisjoint equational theories. Keywords: constraint programming, decision procedure, satisfiability, combination problem (R'esum'e : tsvp) INRIALorraine & CRIN, email: Christophe.Ringeissen@loria.fr Unit de recherche INRIA Lorraine Technpole de NancyBrabois, Campus scientifique, 615 rue de Jardin Botanique, BP 101, 54600 VILLE...
Combination Techniques and Decision Problems for Disunification
 Theoretical Computer Science
"... Previous work on combination techniques considered the question of how to combine unification algorithms for disjoint equational theories E 1 ; : : : ; E n in order to obtain a unification algorithm for the union E 1 [ : : : [ E n of the theories. Here we want to show that variants of this method m ..."
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Cited by 21 (6 self)
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Previous work on combination techniques considered the question of how to combine unification algorithms for disjoint equational theories E 1 ; : : : ; E n in order to obtain a unification algorithm for the union E 1 [ : : : [ E n of the theories. Here we want to show that variants of this method may be used to decide solvability and ground solvability of disunification problems in E 1 [ : : : [E n . Our first result says that solvability of disunification problems in the free algebra of the combined theory E 1 [ : : : [E n is decidable if solvability of disunification problems with linear constant restrictions in the free algebras of the theories E i (i = 1; : : : ; n) is decidable. In order to decide ground solvability (i.e., solvability in the initial algebra) of disunification problems in E 1 [ : : : [ E n we have to consider a new kind of subproblem for the particular theories E i , namely solvability (in the free algebra) of disunification problems with linear constant restricti...
Unification in a combination of equational theories with shared constants and its application to Primal Algebras
 In Proceedings of the 1st International Conference on Logic Programming and Automated Reasoning, St. Petersburg (Russia), volume 624 of Lecture Notes in Artificial Intelligence
, 1992
"... . We extend the results on combination of disjoint equational theories to combination of equational theories where the only function symbols shared are constants. This is possible because there exist finitely many proper shared terms (the constants) which can be assumed irreducible in any equational ..."
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Cited by 16 (4 self)
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. We extend the results on combination of disjoint equational theories to combination of equational theories where the only function symbols shared are constants. This is possible because there exist finitely many proper shared terms (the constants) which can be assumed irreducible in any equational proof of the combined theory. We establish a connection between the equational combination framework and a more algebraic one. A unification algorithm provides a symbolic constraint solver in the combination of algebraic structures whose finite domains of values are non disjoint and correspond to constants. Primal algebras are particular finite algebras of practical relevance for manipulating hardware descriptions. 1 Introduction The combination problem for unification can be stated as follows: given two unification algorithms in two (consistent) equational theories E 1 on T (F 1 ; X) and E 2 on T (F 2 ; X), how to design a unification algorithm for E 1 [ E 2 on T (F 1 [ F 2 ; X)? The ge...
RuleBased Constraint Programming
 Fundamenta Informaticae
, 1998
"... In this paper we present a view of constraint programming based on the notion of rewriting controlled by strategies. We argue that this concept allows us to describe in a unified way the constraint solving mechanism as well as the metalanguage needed to manipulate the constraints. This has the a ..."
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Cited by 8 (1 self)
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In this paper we present a view of constraint programming based on the notion of rewriting controlled by strategies. We argue that this concept allows us to describe in a unified way the constraint solving mechanism as well as the metalanguage needed to manipulate the constraints. This has the advantage to provide descriptions that are very close to the proof theoretical setting used now to describe constraint manipulations like unification or numerical constraint solving. We examplify the approach by presenting examples of constraint solvers descriptions and combinations written in the ELAN language. 1
General A and AXUnification via Optimized Combination Procedures
, 1991
"... In a recent paper [BS91] we introduced a new unification algorithm for the combination of disjoint equational theories. Among other consequences we mentioned (1) that the algorithm provides us with a decision procedure for the solvability of general A and AIunification problems and (2) that Kapur ..."
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Cited by 6 (3 self)
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In a recent paper [BS91] we introduced a new unification algorithm for the combination of disjoint equational theories. Among other consequences we mentioned (1) that the algorithm provides us with a decision procedure for the solvability of general A and AIunification problems and (2) that Kapur and Narendran's result about the NPdecidability of the solvability of general AC and ACIunification problems (see [KN91]) may be obtained from our results. In [BS91] we did not give detailled proofs for these two consequences. In the present paper we will treat these problems in more detail. Moreover, we will use the two examples of general A and AIunification for a case study of possible optimizations of the basic combination procedure.
Combination of Matching Algorithms
 Proceedings 11th Annual Symposium on Theoretical Aspects of Computer Science, Caen (France), volume 775 of Lecture Notes in Computer Science
, 1994
"... . This paper addresses the problem of systematically building a matching algorithm for the union of two disjoint equational theories. The question is under which conditions matching algorithms in the single theories are sufficient to obtain a matching algorithm in the combination? In general, the bl ..."
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Cited by 5 (0 self)
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. This paper addresses the problem of systematically building a matching algorithm for the union of two disjoint equational theories. The question is under which conditions matching algorithms in the single theories are sufficient to obtain a matching algorithm in the combination? In general, the blind use of combination techniques introduces unification. Two different restrictions are considered in order to reduce this unification to matching. First, we show that combining matching algorithms (with linear constant restriction) is always sufficient for solving a pure fragment of combined matching problems. Second, we present a combined matching algorithm which is complete for the largest class of theories where unification is not needed, including collapsefree regular theories and linear theories. 1 Introduction The process of matching is crucial in term rewriting, from automated deduction involving simplification rules to the implementation of operational semantics for programming l...
Combining Unification and Disunification Algorithms  Tractable and Intractable Instances
, 1996
"... We consider the problem of combining procedures that decide solvability of (dis)unification problems over disjoint equational theories. Partial answers to the following questions are given: ffl Which properties of the component theories imply intractability in the sense that there cannot be a polyn ..."
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Cited by 2 (1 self)
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We consider the problem of combining procedures that decide solvability of (dis)unification problems over disjoint equational theories. Partial answers to the following questions are given: ffl Which properties of the component theories imply intractability in the sense that there cannot be a polynomial combination algorithm, assuming P 6= NP? ffl Which general properties of the component theories guarantee tractability of the combination problem in the sense that there exists a deterministic and polynomial combination algorithm? A criterion is given that characterizes a large class K of equational theories E where general Eunification is always NPhard. We show that all regular equational theories E that contain a commutative or an associative function symbol belong to K. Other examples of equational theories in K concern nonregular cases as well. The combination algorithm described in [BS92] can be used to reduce solvability of general Eunification algorithms to solvability of...