Abstract. In this paper we define an inductive set that is bijective with the ff-equated lambda-terms. Unlike de-Bruijn indices, however, our inductive definition includes names and reasoning about this definition is very similar to informal reasoning on paper. For this we provide a structural induction principle that requires to prove the lambda-case for fresh binders only. The main technical novelty of this work is that it is compatible with the axiom-of-choice (unlike earlier nominal logic work by Pitts et al); thus we were able to implement all results in Isabelle/HOL and use them to formalise the standard proofs for Church-Rosser and strongnormalisation. Keywords. Lambda-calculus, nominal logic, structural induction, theoremassistants.

Abstract. We present five axioms of name-carrying lambda-terms identified up to alpha-conversion—that is, up to renaming of bound variables. We assume constructors for constants, variables, application and lambdaabstraction. Other constants represent a function Fv that returns the set of free variables in a term and a function that substitutes a term for a variable free in another term. Our axioms are (1) equations relating Fv and each constructor, (2) equations relating substitution and each constructor, (3) alpha-conversion itself, (4) unique existence of functions on lambda-terms defined by structural iteration, and (5) construction of lambda-abstractions given certain functions from variables to terms. By building a model from de Bruijn’s nameless lambda-terms, we show that our five axioms are a conservative extension of HOL. Theorems provable from the axioms include distinctness, injectivity and an exhaustion principle for the constructors, principles of structural induction and primitive recursion on lambda-terms, Hindley and Seldin’s substitution lemmas and

Abstract. Combining Higher Order Abstract Syntax (HOAS) and induction is well known to be problematic. We have implemented a tool called Hybrid, within Isabelle HOL, which does allow object logics to be represented using HOAS, and reasoned about using tactical theorem proving in general and principles of (co)induction in particular. In this paper we describe Hybrid, and illustrate its use with case studies. We also provide some theoretical adequacy results which underpin our practical work. 1 Introduction Many people are concerned with the development of computing systems which can be used to reason about and prove properties of programming languages. However, developing such systems is not easy. Difficulties abound in both practical implementation and underpinning theory. Our paper makes both a theoretical and practical contribution to this research area. More precisely, this paper concerns how to reason about object level logics with syntax involving variable binding--note that a programming language can be presented as an example of such an object logic. Our contribution is the provision of a mechanized tool, Hybrid, which has been coded within Isabelle HOL, and- provides a form of logical framework within which the syntax of an object

this paper appears in Types for Proofs and Programs: International Workshop TYPES'93, Nijmegen, May 1993, Selected Papers, LNCS 806. abstraction, compute a type for its body in an extended context; to compute a type for an application, compute types for its left and right components, and check that they match appropriately. Lets use the algorithm to compute a type for a = [x:ø ][x:oe]x. FAILURE: no rule applies because x 2 Dom (x:ø )

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Barendregt’s variable convention simplifies many informal proofs in the λ-calculus by allowing the consideration of only those bound variables that have been suitably chosen. Barendregt does not give a formal justification for the variable convention, which makes it hard to formalise such informal proofs. In this paper we show how a form of the variable convention can be built into the reasoning principles for rule inductions. We give two examples explaining our technique.

Abstract. The use of higher-order abstract syntax is an important approach for the representation of binding constructs in encodings of languages and logics in a logical framework. Formal meta-reasoning about such object languages is a particular challenge. We present a mechanism for such reasoning, formalized in Coq, inspired by the Hybrid tool in Isabelle. At the base level, we define a de Bruijn representation of terms with basic operations and a reasoning framework. At a higher level, we can represent languages and reason about them using higher-order syntax. We take advantage of Coq’s constructive logic by formulating many definitions as Coq programs. We illustrate the method on two examples: the untyped lambda calculus and quantified propositional logic. For each language, we can define recursion and induction principles that work directly on the higher-order syntax. 1

: The ß-calculus is a process algebra for modelling concurrent systems in which the pattern of communication between processes may change over time. This paper describes the results of preliminary work on a definitional formal theory of the ß-calculus in higher order logic using the HOL theorem prover. The ultimate goal of this work is to provide practical mechanized support for reasoning with the ß-calculus about applications. Introduction The ß-calculus [17, 18] is a process algebra proposed by Milner, Parrow and Walker for modelling concurrent systems in which the pattern of interconnection between processes may change over time. This paper describes work on a mechanized formal theory of the ß-calculus in higher order logic using the HOL theorem prover [8]. The main aim of this work is to construct a practical and sound theorem-proving tool to support reasoning about applications using the ß-calculus, as well as metatheoretic reasoning about the ß-calculus itself. Four general prin...

Abstract. Placing our result in a web of related mechanised results, we give a direct proof that the de Bruijn λ-calculus (à la Huet, Nipkow and Shankar) is isomorphic to an α-quotiented λ-calculus. In order to establish the link, we introduce an “index-carrying ” abstraction mechanism over de Bruijn terms, and consider it alongside a simplified substitution mechanism. Relating the new notions to those of the α-quotiented and the proper de Bruijn formalisms draws on techniques from the theory of nominal sets. 1

We present a formalization of a typed pi-calculus in the Calculus of Inductive Constructions. We give the rules for type-checking and for evaluation and formalize a proof of type preservation in the Coq system. The encoding of the pi-calculus in Coq uses Coq fonctions to represent bindings of variables. This kind of encoding is called a higher-order specication. It provides a concise description of the calculus, leading to simple proofs. The specification we propose for the pi-calculus formalizes communication by means of function application.