Abstract
Many applications of automated deduction require reasoning in first-order logic modulo background theories, in particular some form of integer arithmetic. A major unsolved research challenge is to design theorem provers that are “reasonably complete” even in the presence of free function symbols ranging into a background theory sort. The hierarchic superposition calculus of Bachmair, Ganzinger, and Waldmann already supports such symbols, but, as we demonstrate, not optimally. This paper aims to rectify the situation by introducing a novel form of clause abstraction, a core component in the hierarchic superposition calculus for transforming clauses into a form needed for internal operation. We argue for the benefits of the resulting calculus and provide two new completeness results: one for the fragment where all background-sorted terms are ground and another one for a special case of linear (integer or rational) arithmetic as a background theory.
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Notes
- 1.
Without loss of generality we assume that there exists a distinct sort for every predicate.
- 2.
This restriction to term-generated interpretations as models is possible since we are only concerned with refutational theorem proving, i. e., with the derivation of a contradiction.
- 3.
To satisfy the technical requirement that all interpretations in \(\mathcal {B}\) are term-generated, we assume that in this case \(\varSigma _\mathrm {B}\) is suitably extended by an infinite set of constants (or by one constant and one unary function symbol) that are not used in any input formula or theory axiom.
- 4.
If \(\varSigma = \varSigma _\mathrm {B}\), this definition coincides with the definition of satisfiability w.r.t. a class of interpretations that was given in Sect. 2. A set N of BG clauses is \(\mathcal {B}\)-satisfiable if and only if some interpretation of \(\mathcal {B}\) is a model of N.
- 5.
- 6.
Note that J need not be a \(\mathcal {B}\)-interpretation.
- 7.
Target terms are terms that need to be abstracted out; so for efficiency reasons, it is advantageous to keep the number of target terms as small as possible. We will show in Sect. 7 why domain elements may be treated differently from other non-variable terms. On the other hand, all the results in the following sections continue to hold if the restriction that q is not a domain element is dropped (i. e., if domain elements are abstracted out as well). We will make use of this fact in Sect. 11.
- 8.
As in [5], it is possible to strengthen the maximality condition by requiring that there exists some simple ground substitution \(\psi \) such that \((s \not \approx t)\sigma \psi \) is maximal in \((s \not \approx t \,\vee \, C)\sigma \psi \) (and analogously for the other inference rules).
- 9.
- 10.
Typically, \(\mathrm {E}_I\) contains two kinds of clauses, namely clauses that evaluate non-constant BG terms, such as \(2 + 3 \approx 5\), and clauses that map parameters to domain elements, such as \(\alpha \approx 4\).
- 11.
In [21], it is required that every ground term that contains a (proper or improper) BG-sorted FG subterm must be larger than any (BG or FG) ground term that does not contain such a subterm.
- 12.
Condition (i) of is obviously satisfied and condition (iii) there is condition (v) of . Instead of condition (ii), inferences are only \(\mathcal {B}\)-satisfiability preserving, which however does not endanger soundness.
- 13.
A KBO with appropriate weights can be used for \(\succ _{\mathrm {fin}}\).
- 14.
Any pair of terms s, t is related in all clauses of an equivalence class by either a literal \(s \approx t\), or \(s \not \approx t\), or \(s <_n t\) for some n, or \(t <_n s\) for some n, or no literal at all, so there are five possibilities per unordered pair of terms.
- 15.
Beagle is available at https://bitbucket.org/peba123/beagle. The distribution includes the (Scala) source code and a ready-to-run Java jar-file.
- 16.
- 17.
SWW583=2.p, SWW594=2.p, SWW607=2.p, SWW626=2.p, SWW653=2.p and SWW657=2.p.
- 18.
At the time of this writing, there are only four provers (including Beagle) registered with the TPTP web infrastructure that can solve these problems. Hence the rating 0.75.
- 19.
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Baumgartner, P., Waldmann, U. (2019). Hierarchic Superposition Revisited. In: Lutz, C., Sattler, U., Tinelli, C., Turhan, AY., Wolter, F. (eds) Description Logic, Theory Combination, and All That. Lecture Notes in Computer Science(), vol 11560. Springer, Cham. https://doi.org/10.1007/978-3-030-22102-7_2
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