Abstract
Leo-III is an automated theorem prover for extensional type theory with Henkin semantics and choice. Reasoning with primitive equality is enabled by adapting paramodulation-based proof search to higher-order logic. The prover may cooperate with multiple external specialist reasoning systems such as first-order provers and SMT solvers. Leo-III is compatible with the TPTP/TSTP framework for input formats, reporting results and proofs, and standardized communication between reasoning systems, enabling, e.g., proof reconstruction from within proof assistants such as Isabelle/HOL. Leo-III supports reasoning in polymorphic first-order and higher-order logic, in many quantified normal modal logics, as well as in different deontic logics. Its development had initiated the ongoing extension of the TPTP infrastructure to reasoning within non-classical logics.
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Note the different capitalization of Leo-III as opposed to LEO-I and LEO-II. This is motivated by the fact that Leo-III is designed to be a general-purpose system rather than a subcomponent to another system. Hence, the original capitalization derived from the phrase “Logical Engine for Omega” (LEO) is not continued.
See the individual projects related to the Leo prover family at https://github.com/leoprover. Further information is available at http://inf.fu-berlin.de/~lex/leo3.
As a simple counterexample, consider a (strict) term ordering \(\succ \) for HOL terms that satisfies the usual properties from first-order superposition (e.g., the subterm property) and is compatible with \(\beta \)-reduction. For any non-empty signature \(\Sigma \), \(\mathrm {c} \in \Sigma \), the chain \(\mathrm {c} \equiv _\beta (\uplambda X.\, \mathrm {c}) \; \mathrm {c} \succ \mathrm {c}\) can be constructed, implying \(\mathrm {c} \succ \mathrm {c}\) and thus contradicting irreflexivity of \(\succ \). Note that \((\uplambda X.\, \mathrm {c}) \; \mathrm {c} \succ \mathrm {c}\) since the right-hand side is a proper subterm of the left-hand side (assuming an adequately lifted definition of subterm property to HO terms).
The Identity of Indiscernibles (also known as Leibniz’s law) refers to a principle first formulated by Gottfried Leibniz in the context of theoretical philosophy [71]. The principle states that if two objects X and Y coincide on every property P, then they are equal, i.e., \(\forall X_\tau .\,\forall Y_\tau .\,\left( \forall P_{o\tau }. P \; X \Leftrightarrow P \; Y\right) \Rightarrow X = Y\), where “\(=\)” denotes the desired equality predicate. Since this principle can easily be formulated in HOL, it is possible to encode equality in higher-order logic without using the primitive equality predicate. An extensive analysis of the intricate differences between primitive equality and defined notions of equality is presented by Benzmüller et al. [22] to which the authors refer for further details.
In fact, the implementation of Leo-III employs a nameless representation of bound variables such that variable capture is avoided by design, cf. §4.5 for details.
The set \(\mathcal {GB}_{\tau }^{t}\) of approximating/partial bindings parametric to a type \(\tau = {\beta {\alpha ^n\ldots \alpha ^1}}\) (for \(n\ge 0\)) and to a constant t of type \({\beta {\gamma ^m\ldots \gamma ^1}}\) (for \(m\ge 0\)) is defined as follows (for further details see also [88]): Given a “name” k (where a name is either a constant or a variable) of type \({\beta {\gamma ^m\ldots \gamma ^1}}\), the term l having form \(\uplambda {X^1_{\alpha ^{1}}. \ldots \uplambda X^n_{\alpha ^{n}}}. (k\, {{r^1}\ldots {r^m}})\) is a partial binding of type \({\beta {\alpha ^n\ldots \alpha ^1}}\) and head k. Each \({r}^{i \le m}\) has form \(H^i {X^1_{\alpha ^1}\ldots X^n_{\alpha ^n}}\) where \(H^{i\le m}\) are fresh variables typed \(\gamma ^{i\le m}{\alpha ^n\ldots \alpha ^1}\). Projection bindings are partial bindings whose head k is one of \(X^{i\le l}\). Imitation bindings are partial bindings whose head k is identical to the given constant symbol t in the superscript of \(\mathcal {GB}_{\tau }^{t}\). \(\mathcal {GB}_{\tau }^{t}\) is the set of all projection and imitation bindings modulo type \(\tau \) and constant t. In our example derivation we twice use imitation bindings of form \(\uplambda X_\iota . \lnot (H_{o\iota } X_\iota )\) from \(\mathcal {GB}_{o\iota }^{\lnot }\).
Leo’s Parallel Architecture and Datastructures (LeoPARD) can be found at https://github.com/leoprover/LeoPARD.
Remark on CAX: In this special case of THM (theorem), the given axioms are inconsistent so that anything follows, including the given conjecture. Hence, it is counted against solved problems.
This information is extracted from the TPTP problem rating information that is attached to each problem. The unsolved problems are NLP004⌃7, SET013⌃7, SEU558⌃1, SEU683⌃1, SEV143⌃5, SYO037⌃1, SYO062⌃4.004, SYO065⌃4.001, SYO066⌃4.004, MSC007⌃1.003.004, SEU938⌃5 and SEV106⌃5.
HOLyHammer (HOL ATP) and Zipperposition (first-order ATP) are the only other systems supporting polymorphism.
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This work has been supported by the DFG under Grant BE 2501/11-1 (Leo-III) and by the Volkswagenstiftung (“Consistent Rational Argumentation in Politics”).
Leo-III Proof of Fig. 7
Leo-III Proof of Fig. 7
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Steen, A., Benzmüller, C. Extensional Higher-Order Paramodulation in Leo-III. J Autom Reasoning 65, 775–807 (2021). https://doi.org/10.1007/s10817-021-09588-x
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DOI: https://doi.org/10.1007/s10817-021-09588-x