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Property Conveyances As a Programming Language

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Property Conveyances As a Programming Language Property Conveyances as a Programming Language Shrutarshi Basu∗ Nate Foster James Grimmelmann Harvard University Cornell University Cornell Law School & Cornell Tech Cambridge, MA, USA Ithaca, NY, USA New York, NY, USA [email protected] [email protected] [email protected] Abstract 1 Introduction Anglo-American law enables property owners to split up Many legal issues depend on vague, open-ended standards. rights among multiple entities by breaking their ownership For example, nuisance law prohibits “unreasonable” interfer- apart into future interests that may evolve over time. The con- ence with neighbors’ use of land. What counts as “unreason- veyances that owners use to transfer and subdivide property able” depends on an eight-factor balancing test, and many of rights follow rigid syntactic conventions and are governed the individual factors are themselves vague and open-ended, by an intricate body of interlocking doctrines that determine including “the social value that the law attaches to the pri- their legal eect. These doctrines have been codied, but mary purpose of the conduct;” and “the suitability of the only in informal and potentially ambiguous ways. particular use or enjoyment invaded to the character of the This paper presents preliminary work in developing a locality.” [American Law Institute 1979] A lawyer who wants formal model for expressing and analyzing property con- to know whether a client’s cement plant will be considered veyances. We develop a domain-specic language capable a nuisance will have to research numerous previous cases, of expressing a wide range of conveyances in a syntax ap- gather extensive facts about the plant and its neighbors, and proximating natural language. This language desugars into a rely on her accumulated intuitions about how courts tend core calculus for which we develop operational and denota- to rule. Indeed, whether her client can build the plant may tional semantics capturing a variety of important properties depend on the lawyer’s skill in presenting an interpretation of property law in practice. We evaluate an initial implemen- of the facts and the factors that favors the client. tation of our languages and semantics on examples from a Other legal issues depend on clearer and relatively unam- popular property law textbook. biguous rules. For example, the tax code generally allows individuals to deduct the home mortgage interest they pay, CCS Concepts • Applied computing → Law; • Software but only on loans of up to $750,000. There is no room to and its engineering → General programming languages; argue over whether “$750,000” really should mean $75,000, • Theory of computation → Denotational semantics; Op- $75,000,000, or a context-dependent amount. Tax law is full erational semantics; • Human-centered computing → In- of unambiguous, mechanical calculations—this is why tax teractive systems and tools. software is possible. These portions of tax law are eectively Keywords Domain-Specic Languages, Law, Semantics formalizable [Lawsky 2016]. In other areas of law, however, lawyers have been less ACM Reference Format: Shrutarshi Basu, Nate Foster, and James Grimmelmann. 2019. Prop- quick to appreciate that the rules they work with have an erty Conveyances as a Programming Language. In Proceedings of underlying logical structure. Indeed, legal training, with its the 2019 ACM SIGPLAN International Symposium on New Ideas, New strong emphasis on careful reading and rhetorical skills, and Paradigms, and Reections on Programming and Software (Onward! its corresponding lack of emphasis on quantitative and for- ’19), October 23–24, 2019, Athens, Greece. ACM, New York, NY, USA, mal methods, may actively discourage lawyers from thinking 15 pages. hps://doi.org/10.1145/3359591.3359734 about legal rules in symbolic, rather than verbal, terms. Legal analysis tends to be inductive, rather than deductive, even ∗This work was started while the author was at Cornell University. where the law itself allows for truly deductive reasoning. Permission to make digital or hard copies of all or part of this work for The elegant structure of the rules themselves is obscured personal or classroom use is granted without fee provided that copies when they are analyzed with the open-ended techniques one are not made or distributed for prot or commercial advantage and that might use to argue whether a land use is ‘reasonable.” copies bear this notice and the full citation on the rst page. Copyrights One such body of rules is the system of “estates in land” for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or that govern the division of property rights over time. It is republish, to post on servers or to redistribute to lists, requires prior specic incredibly common for ownership of real estate (i.e. land permission and/or a fee. Request permissions from [email protected]. and buildings) to be divided among multiple people and for Onward! ’19, October 23–24, 2019, Athens, Greece the passage of time and the occurence of various events to © 2019 Copyright held by the owner/author(s). Publication rights licensed change the division. For example, in a lease, both the landlord to ACM. and the tenant have interests in the property, each of which ACM ISBN 978-1-4503-6995-4/19/10...$15.00 hps://doi.org/10.1145/3359591.3359734 comes with dierent rights. The tenant has the present right 128 Onward! ’19, October 23–24, 2019, Athens, Greece Shrutarshi Basu, Nate Foster, and James Grimmelmann to possess the property now; the landlord is not entitled Some earlier work by the authors [Basu et al. 2017] described to possession, but does have the right to receive periodic how the language of conveyances could be boiled down to an rent payments from the tenant. In the future, when the lease abstract syntax that allowed for some automated reasoning. term ends, the tenant’s right to possession will end, and the But we did not provide semantics or derive interesting prop- landlord will be entitled to reenter the property and retake erties of our formalism. This paper provides a more formal possession. Or, the lease might provide that if a specied treatment of estates in land through the lens of programming event occurs—for example, if an apartment tenant causes language theory. Our contributions are as follows: serious damage to the building—the tenant’s interest might terminate immediately. • We develop a domain-specic language that is capable Law students learn the basics of the dierent types of of expressing a wide range of realistic conveyances. It estates in land in their property course, which is typically does not capture every turn of phrase a lawyer might taught in the rst year. They learn how to parse the legal use, but it covers the standard stock phrases that rst- documents (generically known as conveyances) by which year law students become familiar with. one person can give her interest to another, or divide it up • The surface syntax for this language is ambiguous in among several others, and how to give legally-meaningful ways that reect well-known ambiguities in the nat- descriptions of the resulting interests (such as “remainder in ural language of actual conveyances. However, the life estate subject to an executory limitation,” meaning that ambiguities are limited in scope. We use a parser com- one has the right to a piece for property for their lifetime, binator library [Arnold and Mouratov 2008] to imple- as long as they satisfy some additional condition). It takes ment a parser for the language, one that covers a range several weeks to a month of class time and is frequently of examples from a popular property law textbook. regarded as a tedious and pointless exercise in memoriz- • We develop a minimal core calculus that can express ing arcane rules [Grimmelmann 2017]. At the end of that how the interests dened by a conveyance change time, students are hopefully able to read a conveyance such in response to a sequence of events. Conveyances in as “O conveys to A for life, then to B for life, then the surface language desugar deterministically into a to C for life.” and report that it creates four interests: program in this core calculus. 1. A has a life estate. • We develop an operational and a denotational seman- 2. B has a remainder in life estate. tics for programs in the core calculus and prove their 3. C has a remainder in life estate. equivalence. The operational semantics lead to a straight- 4. O has a reversion in fee simple. forward functional implementation. The denotational A “life estate” means that the interest is valid until the semantics allow us to derive useful properties of the death of the possessor, while “fee simple” means that the language, and to give succinct proofs of several max- interest is valid indenitely (and passes to the possessor’s ims used by lawyers to describe the functioning of the legal heirs on their death). As previous legal scholars have estates in land (e.g. “First in time, rst in right.”). noted, this example suggests the grammar of conveyances has a recursive structure: adding an additional clause (then The rest of this paper is organized as follows: to D for life) would create an additional interest (a remainder in life estate for D) [Edwards 2009; Merrill and Smith 2017]. 1. Section 2 gives a number of examples of conveyances, Other conveyances have subtle consequences that are not explains the relevant terminology, and shows how they obvious from a layperson’s reading. Consider “O conveys to may be interpreted as programs in a formal language. A for life, then to B for life, but if C marries then 2.
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