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Abstract: Abstract solvers are a tool for describing solving algorithms via graphs, that entail some advantages w.r.t. traditional ways of describing solving algorithms, e.g. pseudo-code. In the letter, first I briefly mention the main ideas and advantages of abstract solvers. Then, I give useful pointers to the usage of abstract solvers in Answer Set Programming, and beyond.
Abstract solvers are a relatively new tool for describing solving algorithms for given reasoning tasks. Usually, solving algorithms are presented by means of pseudo-code descriptions, but some communities have experienced that analyzing algorithms on this basis may not be fruitful. Thus, more formal descriptions which allow, e.g., for a uniform representation, have been studied and developed. In abstract solvers, such desiderata are met by relying on graphs: one characterizes the states of computation as nodes of a graph, the techniques as arcs between nodes, the whole solving process as a path in the graph, and formal properties of the algorithms are reduced to related graph’s properties. Thus, abstract solvers (i) allow for a more deep understanding of the ideas behind a solving procedure, (ii) simplify stating and proving formal properties of the related algorithms, e.g. correctness, and (iii) support the comparison of different search procedures by means of comparing their related graphs.
The use of abstract solvers was initiated by Nieuwenhuis et al. in . In this work the authors first presented an abstract solver related to the DPLL procedure for SAT. Then, they presented two extensions to this graph: (i) a first extended graph to describe CDCL SAT solving, i.e., involving backjumping and learning techniques, by means of modular addition of transition rules, and (ii) a second graph to solve Satisfiability Modulo Theories (SMT) problems with certain logics via a lazy approach , i.e., where the SAT calls are made to provide satisfying assignments of the Boolean abstraction of the SMT problem that are then checked for “SMT consistency.” Lierler  then imported this methodology in ASP, by first designing abstract solvers for some backtracking-based ASP solvers for non-disjunctive ASP solving, then enhanced to include backjumping and learning techniques . Another extension for describing CASP solvers, i.e. systems able to deal with a combination of ASP and constrain programming, a language useful to represent and reasoning on hybrid domains, has been put forward in . Other papers on abstract solvers are , where solvers for different formalisms, i.e. ASP and SAT with Inductive Definitions, are compared by means of comparison of the related graphs. Brochenin et al.  exploited abstract solvers for cautious reasoning in ASP.
Abstract solvers for some disjunctive answer set solvers implementing basic backtracking have been introduced by Brochenin et al., and then showed, by the same authors, to be instantiations of a more general graph template that includes the techniques implemented by these solvers in .
Application of abstract solvers outside ASP include Quantified Boolean Formulas  and certain reasoning tasks of Dung’s Argumentation Frameworks . Moreover, starting from a developed concept of modularity in answer set solving , abstract modeling of solvers for multi-logic systems are presented in [11-13].
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2. Brochenin, R., Lierler, Y., Maratea, M.: Abstract disjunctive answer set solvers via templates. In: Theory and Practice of Logic Programming. To appear (2016)
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