Nominal Terms and Nominal Logics: From Foundations to Meta-mathematics

Abstract

Nominal techniques concern the study of names using mathematical semantics. Whereas in much previous work names in abstract syntax were studied, here we will study them in meta-mathematics. More specifically, we survey the application of nominal techniques to languages for unification, rewriting, algebra, and first-order logic.What characterises the languages of this chapter is that they are first-order in character, and yet they can specify and reason on names. In the languages we develop, it will be fairly straightforward to give first-order ‘nominal’ axiomatisations of name-related things like alpha-equivalence, capture-avoiding substitution, beta- and eta-equivalence, first-order logic with its quantifiers—and as we shall see, also arithmetic. The formal axiomatisations we arrive at will closely resemble ‘natural behaviour’; the specifications we see typically written out in normal mathematical usage.This is possible because of a novel name-carrying semantics in nominal sets, through which our languages will have name-permutations and term-formers that can bind as primitive built-in features.This chapter draws together material from several papers to deliver a coherent account of a journey from the foundations of a mathematics with names, via logical systems based on those foundations, to concrete applications in axiomatising systems with binding. Definitions and proofs have been improved, generalised, and shortened, and placed into an overall narrative.On the way we touch on a variety of definitions and results. These include: the nominal unification algorithm; nominal rewriting and its confluence proofs; nominal algebra, its soundness, completeness, and an HSP theorem; permissive-nominal logic and its soundness and completeness; various axiomatisations with pointers to proofs of their correctness; and we conclude with a case study stating and proving correct a finite first-order axiomatisation of arithmetic in permissive-nominal logic.

Appendices

Appendix

A An Atlas of Nominal Languages

The reader coming to the nominal literature could be forgiven for finding it perplexing. What are ‘Fraenkel-mostowski sets’, ‘nominal sets’, ‘nominal terms’, ‘nominal logic’, ‘nominal rewriting and algebra’, ‘αProlog’, ‘nominal equational logic’, ‘permissive-nominal algebra’, ‘permissive-nominal logic’ (with/without shift-permutations)? In this Appendix we will give a brief annotated bibliography covering, loosely, the relevant publications. This list is not meant to be exhaustive.

Traditionally, nominal sets are understood as a tool for the mathematical analysis of syntax, as described for instance in the author’s previous survey/research paper Gabbay (2011b), or in slides of an excellent course of lectures by Pitts (2011). This author takes a view of nominal sets not just as a foundation for syntax with binding, but as a foundation for mathematics itself—names and binding, after all, appear everywhere. The atlas below surveys relevant publications.

For each item in the list below, we reference where the idea was introduced to the ‘nominal’ literature, and any other relevant conference and journal papers.

2.2.1 A.1 FM Set Theory (Gabbay and Pitts 1999, 2001)

Fraenkel-Mostowski set theory (FM) and nominal sets (called ‘equivariant FM sets’ in that paper) are the foundational semantics for nominal techniques.

Fraenkel-Mostowski sets were already known and had been used for other purposes; see (Gabbay, 2011b, Remark 2.22) for more detailed historical comments. Nominal sets were familiar as e.g. the Schanuel topos. So both semantics were known.

What was new to Gabbay and Pitts (2001) was the observation by the author and Pitts of the notions of support, atoms-abstraction, the self-dual behaviour of the

quantifier, and the application to what is now called nominal abstract syntax.²¹

2.2.2 A.2 Nominal Logic (Pitts 2001, 2003)

The constructions of Gabbay and Pitts (2001) are repeated, but in a first-order axiomatisation of nominal sets rather than one of the FM cumulative hierarchy. Pitts also coined the catchy label ‘nominal’.

Sometimes authors identify the nominal logic of Pitts (2003) with nominal techniques in general. This is limiting, and it gets the mathematical development the wrong way round. Nominal logic is a Hilbert-style axiomatisation in first-order logic. These axioms have meaning because of the underlying nominal sets models, and not the other way around; nor does the axiomatisation per se contribute to new syntax or proof-theory with which to study names.

In order to make progress, we needed new syntax that more explicitly represents atoms and their properties.

Thus for instance the nominal logic programming developed by Cheney and Urban (2008) (also referenced below) is called logic-programming in nominal logic, but we also see from Figures 6, 7, and 8 of Cheney and Urban (2008) that the syntax and axioms used are a variant of nominal terms.

2.2.3 A.3 Proof-Theories for the -quantifier (Gabbay 2007a; Gabbay and Cheney 2004; Cheney 2005b)

Some attempts have been made to give the distinctive

-quantifier of nominal techniques, a proof-theory. In arguably increasing order of elegance these are Gabbay (2007a) (this was received by the journal in 2003 but took 4 years to get printed), Gabbay and Cheney (2004) (written with Cheney to develop on Gabbay (2007a)), and Cheney (2005b).

The permissive-nominal logic (PNL) of this survey is another item on that list, and perhaps it is one of the nicest; certainly the PNL treatment of

is very different from what has come before, see Sect. 2.4.3.1.

Complete semantics for this family of logics are in Gabbay (2007a), Cheney (2006), and in Dowek and Gabbay (2012a). See also Sect. 2.4.1.4 of this survey.

2.2.4 A.4 Nominal Terms (Urban et al. 2003, 2004)

This new syntax introduced the distinctive freshness side-conditions and the nominal terms syntax, with its separation of atoms a and unknowns X into two syntactic classes. Urban et al. (2004) is where the syntactic ideas of this survey were born, if not the specific ‘permissive’ implementation, which came later (permissive-nominal terms below).

There is now quite a substantial body of work devoted to computing efficiently on nominal terms; notably Calvès (2010), Levy and Villaret (2010). There is also a body of work devoted to translating between nominal terms and higher-order patterns Miller et al. (1989). We are far from exhaustive, but good places to start reading are Cheney (2005a), Levy and Villaret (2008, 2012), Gabbay and Mulligan (2009), and Dowek et al. (2010).

2.2.5 A.5 Nominal Rewriting (Fernández et al. 2004; Fernández and Gabbay 2007) and αProlog (Cheney and Urban 2003, 2008)

These were the first logical languages using nominal terms as a general-purpose assertion language; nominal rewriting was designed explicitly to allow us to assert (directed) equalities between terms such as β or η-equivalence. αProlog was intended by its designers for reasoning on nominal abstract syntax, and explicitly presented as such—but in retrospect it can also be viewed as a general-purpose ‘nominal’ reasoning system in the same family as nominal rewriting and later work.²²

2.2.6 A.6 Nominal Algebra (Gabbay 2005; Gabbay and Mathijssen 2006a, 2007, 2009)

Nominal algebra is simply the undirected version of nominal rewriting.²³ What makes nominal algebra interesting above and beyond nominal rewriting is the different theorems we prove about equality instead of rewriting; for instance the HSPA theorem of Gabbay (2009) (much simplified here in Sect. 2.3.5), and various correctness results for axiomatisations of e.g. substitution, λ-calculus, and first-order logic Gabbay and Mathijssen (2006a,c, 2008a,b,c, 2010).

The paper Gabbay and Mathijssen (2006a) is where the permutative convention of Definition 2.2 was introduced, used by the author consistently since then. This comes from the author’s work formalising nominal reasoning in Isabelle in Gabbay (2001) and spares us from having to explicitly enumerate all inequalities between atoms. Thus, if pressed to be entirely formal, ‘a and b’ refers to two meta-variables ranging over distinct atoms.

Kurz and Petrişan proved an HSP theorem for nominal algebra by treating nominal algebra as a kind of many-sorted first-order logic Kurz and Petrişan (2010)—the sorts are finite sets of atoms and come from the categorical view of nominal sets as presheaves. The effect of nominal theories can thus be attained in many-sorted first-order syntax. That syntax is just standard first-order syntax is potentially a big advantage, for instance if one wants to transfer results directly from universal algebra. This offers alternative and effective methods of semantic proof; e.g. Kurz and Petrişan (2010) significantly simplifies the proofs of Gabbay (2009). We pay for this convenience with infinities; e.g. even the simplest theory is infinite since equalities are replicated at every sort. Of course, the theory may still be finitely presentable. Section 2.3.5 of the current paper contains another, further simplified, HSP proof.

2.2.7 A.7 Nominal Equational Logic (Clouston and Pitts 2007; Clouston 2011)

Call the judgement ‘a is fresh for the syntax s’ syntactic freshness and ‘a is fresh for the denotation of s’ semantic freshness. Nominal Equational Logic (NEL) closely resembles NA, but whereas both have a semantic equality judgement (s = t), NEL adds a semantic freshness judgement.

In Clouston and Pitts (2007) Clouston and Pitts claimed that NEL was significantly more complete than NA because of this, but they had missed that semantic freshness is expressible using equality and syntactic freshness (see for instance (Gabbay and Mathijssen 2007, Theorem 5.5) and (Gabbay and Mathijssen 2009, Lemma 4.51)).²⁴

Note that two distinct logics have been called NEL: one in Clouston and Pitts (2007), and one in Clouston (2011) which restricts semantic freshness to the left of the turnstile; compare Figure 5 of Clouston and Pitts (2007) with Figure 1 of Clouston (2011). Both have syntactic freshness: see the side-condition $a\#(\bar{a},t,t^{\prime})$ in the Atm-Intro and Atm-Elim rules of Figure 5, and similar side-conditions in Figure 1. Thus, when Clouston writes in Clouston (2011) that “[syntactic] freshness in NA is sound, but not complete, for freshness in the underlying nominal sets interpretation [semantic freshness]”, echoing similar comments in Clouston and Pitts (2007), this omits mention that NEL also has a syntactic freshness.

It is in any case a red herring. If we can choose fresh atoms and compare elements for semantic equality, then semantic freshness makes the logic ‘do equality twice’ and just adds complexity Gabbay (2012b)—without equality, the story can be different; this was encountered in the first attempt at a nominal functional programming language, in which we included freshness information in the types Gabbay (2001).

2.2.8 A.8 Permissive-Nominal Terms (Gabbay and Mulligan 2009; Dowek et al. 2009, 2010)

These simplify and improve classical nominal terms in two ways: we give explicitly the (countably infinite) atoms that may be free/are guaranteed to be fresh in every unknown, and since freshness information is stored directly we eliminate the need for freshness contexts. Thus good properties emerge: permissive-nominal terms can be constructed as nominal abstract syntax, we can directly choose a name fresh for a term (which is not possible in nominal terms without expanding the freshness context), and properties and proofs can then be expressed for terms alone, rather than for terms-in-freshness-context.

For instance, in classical nominal terms a solution of a nominal unification problem is a pair of a substitution and a freshness context; a nominal rewrite rule is a left and a right-hand side term and a freshness context; the proof-theory of nominal algebra requires an explicit freshness rule to generate fresh atoms, and so on. In fact, manipulating nominal terms almost always requires us to manipulate an external structure representing freshness constraints.

In contrast permissive terms are ‘self-sufficient’, like ordinary syntax. Proofs and algorithms have more of the look and feel of ordinary syntax. We have seen how, in the body of this survey. A detailed treatment of permissive-nominal syntax, including a simple translation from the nominal terms of Urban et al. (2004) into permissive-nominal terms, is also in Dowek et al. (2010).

2.2.9 A.9 Permissive-Nominal Algebra (Gabbay and Mulligan 2009, and Sect. 2.3.4)

The permissive-nominal algebra of Sect. 2.3.4 uses permissive-nominal terms and has a significantly different proof-theory.

The notable differences are, aside from being permissive-nominal, the inclusion (if we want them) of infinitely-supported constant symbols and of infinitely-supported permutations. So previous work is a special case of the general framework of this survey, but what we do here goes strictly beyond what was possible in previous work, also in some significant mathematical properties such as satisfying an HSP instead of an HSPA result; see the discussion opening Sect. 2.3.5.

2.2.10 A.10 Permissive-Nominal Logic (Dowek and Gabbay 2010, 2012a and Sect. 2.4.1)

As we discuss in this survey, permissive-nominal logic (PNL) adds universal quantification over unknowns X. This is non-evident for nominal terms because of their freshness contexts; in nominal terms X behaves like an element with cofinite support so we lose α-equivalence whereas in permissive-nominal terms X has coinfinite support and we can always α-rename bound atoms. We get a proof-theory which is pleasingly close to that of first-order logic, a sound and complete semantics, and we can axiomatise and prove correct a non-trivial and mathematically relevant theory, such as arithmetic.

Fig. A.1

Cheat-sheet of nominal languages