Property Law As a Programming Language

Property Law as a Programming Language

Shrutarshi Basu James Grimmelmann Nate Foster Cornell University Cornell Law School Cornell University [email protected] [email protected] [email protected]

Abstract of the formal languages programmers use to instruct computers Anglo-American law enables property owners to split up rights to in how to carry out computations. One of the great achievements a thing over time among multiple people by breaking their own- of the eld has been its development of elegant and rigorous ership apart into multiple future interests. e system of legal doc- semantics that use mathematical tools to explain what a given trines governing future interests is notorious among lawyers and program does and means. Here, we use those tools to describe the law students for its complexity. e conveyances property owners law of future interests. use to transfer and subdivide property rights follow rigid syntactic We do so by treating property conveyances as programs in a conventions and are governed by an intricate body of interlock- very restricted programming language. We show that the stan- ing legal doctrines that determine the legal eect of a conveyance dard terms used to describe and distinguish future interests can over time. ese doctrines have been codied, but only in infor- be represented using a formal syntax we call ConAbs. It has, for mal and potentially ambiguous ways. However, both the syntactic example, a specic representation for a life estate, a dierent rep- structure and underlying semantic of conveyances are amenable to resentation for a condition subsequent, and so on. We then give a analysis using tools and techniques from programming language precise semantics for ConAbs. is semantics is purely formal and theory and practice. is paper presents preliminary work in de- completely unambiguous: a given ConAbs representation always veloping a formal model for expressing and analyzing property behaves exactly the same way under the same circumstances. But conveyances. it captures the relevant legal doctrines, we think correctly. 2. Concepts and Terms 1. Introduction is section denes some concepts and terms that will be useful for Anglo-American property law governs what things can be owned, the rest of the paper. Property law deals with the rights of owners who owns them, what rights owners enjoy, and how these rights of property. For real estate, these rights are termed present estates can be transferred and divided. Some portions of property law, if their owner can currently excercise them, or future interests, if such as nuisance law (governing the relations among neighors) they can only be exercised (if at all) at some point in the future. We use open-ended standards and factually intensive balancing tests. will call any property right capable of being owned an interest. In- But other portions are infamous for their rigidity: they rely on terests are created (and the corresponding rights transferred) from intricate bright-line rules, and lots of them. one party (the grantor) to another (the grantee) in a variety of ways e most rigid, intricate, and bright-line portion of property (wills, deeds, etc.), which we will collectively call conveyances. law is the system of doctrines governing the division of ownership For a rst example, consider the simplest non-trivial con- of property over time. One can, for example, give ownership of veyance, shown in Conveyance 1. a tract of land to one person for her lifetime, and to another person thereaer. Although the system exhibits great exibility O conveys to A (1) in practice, that exibliity is achieved only by way of a dense O tangle of doctrines that restrict the permissible interests in a piece Here, assuming that started out owning the property out- A O of property. right, ends up owning it outright, because conveyed her en- A Frequently, lawyers, law students, and legal scholars regard tire interest to . e property term for this type of ownership is A these restrictive and rule-oriented portions of property law as fee simple absolute. is entitled to possess the property now and essentially arbitrary. Certainly, this is commonly how they are forever, and also to convey it to another grantee if he so desired. A taught: with a lot of rote memorization of denitions and exam- We will say that the nature of ’s interest is “possessory”, and that ples. Compared to the rest of property law, the law of future in- its duration is “fee simple absolute.” terests is regarded as excessively formal: full of fusty distinctions Now, consider the slightly more complicated example shown O A and traps for the unwary. in Conveyance 2. A grantor conveys an interest to a grantee A B Another way of thinking about the problem, though, is that not forever but for a shorter time: ’s lifetime. Aer that, is to A A perhaps this fragment of property law is not too formal but rather receive the property. Before ’s death, is entitled to possess and B not formal enough. e aempts to codify its doctines (most no- use the property; aerwards, is. tably in the 1936–40 Restatement of Property) went a long way toward making it precise, but they still rely on English descrip- O conveys to A for life, then to B (2) tions, and sometimes on highly ambigious descriptions. e un- is conveyance has two clauses (conventially separated by derlying rules, however, are oen well-dened enough that they commas) and it creates two distinct interests. A receives a life can support a genuinely rigorous formal treatment. estate and B receives a remainder in fee simple absolute. e nature In this paper, we give such a treatment, one that uses the tools of A’s interest is still possessory, but now its duration is something of programming language theory. is branch of computer science less than perpetual, it is a “life estate.” It will expire at A’s death. deals with the design, specication, analysis, and implementation B’s interest, on the other hand, has a dierent nature: it is called a “remainder”, because that is the name for an interest that follows and (2) that a condition subsequent can only operate to return pos- a life estate. Once it becomes possessory, however, B’s interest session to the grantor, not to a third party. will entitle B to possess the property forever, so the duration of B’s interest is “fee simple absolute.” As this example shows, an interest’s nature describes how it begins; an interest’s duration To A, but if A divorces then O may retake the property. (6) describes how it ends (if at all). Finally, an interest can be subjected to a condition precedent: In the metaphysics of future interests, A’s death is regarded as even if the interest appears to be next in line, it cannot become 0 causing A life estate to expire naturally. It is also possible to add possessory unless and until the condition is satsied. Conveyance a special limitation (which we will simply call a limitation) to an 7 gives a simple example. Here, A’s life estate is followed by B’s interest, as in Conveyance 3. (From here on, we omit the formulaic remainder in life estate and C’s remainder in fee simple. But B’s “O conveys” when it is clear from context. ) remainder has a condition precedent aached. If B is underage at A’s death, B does not take possession immediately. Instead, in most or all states today, O temporarily recovers possession (via an To A for life so long as A lives on the property, then to B (3) implied reversion) until B turns 21 and the condition precedent is satsied. If B dies before then, B’s interest expires before it be- Here, the limitation is created by the phrase ”so long as A comes possessory, and C’s remainder becomes posessory instead. lives on the property,” and it reduces the duration of A’s interest. Now A holds not a life estate but a life estate determinable. e interest will expire at A’s death or when A stops living on the To A for life, then to B for life if B has turned 21, then to C. property, whichever comes rst. Terminologically, it is helpful to (7) break up durations into two halves. e rst part, which some is is a brief survey of the essential concepts. We will say more writers call the quantum and so do we, is “life estate.” ere are about the details in later sections as we walk through (and justify only a small handful of permissible quanta in property law: fee our choices of) semantic rules. simple, life estate, term of years, and a few more. e second part is the limitation, “so long as A lives on the property.” e full 3. Formalizing Conveyances and Interests duration, then, consists of the interest’s quantum together with the special limitation, if any. Nomenclature aside, “To A for life, then to B” and “To A unless An interest can expire naturally, but it can also be cut short A is disbarred, then to B” have a similar structure: an initial in- by a later interest. Conveyance 4 shows an example of interests terest that expires if an event occurs, followed by a second in- potentially divested by a later interest. terest that becomes possessory if it does. e rest of this section denes a model based on this intuition. Our goal is to develop a model that is exible enough to capture common classes of inter- To A for life, then to B, but if C marries to C (4) ests, and that supports computational analysis. In particular, our model should cover interests with quanta and added limitations, O Here, ’s conveyance creates three interests. If it weren’t for and those subject to conditions precedent and subsequent as well C A the nal clause giving an interest, this would be the same as : as executory limitations. Initially, we would like this model to pro- B has a life estate, and then has a remainder in fee simple. But duce the current possessory interest, given some conveyances and C under the rules of interpreting conveyances, if ever marries, a set of events. With this as a foundation, we can imagine extend- then she will immediately have full right to possess and use the ing the model to handle “possible worlds”: exploring dierent se- divest A B property. Her interest will ’s and ’s interests, regardless quences of events and proving guarantees about the resulting in- A of whether is alive or dead. e full names for the interests here terests (for example, proving that a certain person always, even- A life estate subject to executory limitation B are that has a , has a tually, gains possession). remainder in fee simple subject to executory limitation C , and has a e core elements of our model are laid out in Figure 1a. e shiing executory interest in fee simple . model is based around persons p, events e, and predicates φ or π In several legally signicant ways, the natural expiration of an that are Boolean combinations of events. For a event e to be “true” interest and its premature divestment by a later interest dier. But means that the event has occurred.1 We rst dene ConAbs: an their most important consequence is the same: they immediately abstract language for conveyances that retains legally meaning- terminate the interest, forever destroying any rights its owner held ful information such as natures, durations, limitations (and names under it. and types thereof), but elides the concrete syntactic details. An ree more buliding blocks of future interests are important. interest in ConAbs is a tuple of 4-elements: a precondition ψ, an e rst is that some interests can be implied rather than explicit. owner p, a natural duration δ and an added limitation λ. Note that Conveyance 5 shows why implied interests are sometimes neces- natural durations can be absolute (Abs). No added limitation is O A sary. Here, has given away less than she owns. But if has indicated by leing λ be the predicate “false” (i.e., an event that A only a life estate, what happens to the proeprty aer ’s death? can never occur). Preconditions are richer: they include conditions O e answer is that it goes back to , so this conveyance implies precedent and subsequent, and executory limitations, each with O reversion an interest owned by , an interest called a . a corresponding predicates φ. Interests can be chained together to form lists (using the :: operator). Finally, a conveyance com- To A for life. (5) bines a grantor g with a list of interests i, and multiple such conveyances can be sequenced together. A sequences of conveyances e second building block is that a grantor can explicitly re- in ConAbs can be derived from the corresponding natural lan- serve to herself the right to retake the property if a condition is guage text using a straightforward parser. For now, we elide the breached. Conveyance 6 gives a simple example. is condition details of the parser in this paper to focus on the formalization. subsequent (“but if A divorces then O may retake the property”) can divest a prior interest, as in 4. e dierences are (1) that O 1 We plan to extend our treatment of events and predicates to cover tem- must explicitly act to reclaim the property, whereas with an ex- poral relationships, as in “A dies before B dies” and “alcohol is served on ecutory limitation the property changes ownership automatically, the property aer 2025.” Strings s ::= Strings D · : δ → φ Persons p ::= o, g, a, b . . . Owner, grantor & others D Life(Jp) K = p dies Events e ::= p dies D JTerm(n) K = n years pass | p re-enters J K | n years pass D Abs = false J K | s occurs Predicates π, φ ::= e T · : ConAbs Conveyance → ConCore Tree | ¬φ J K T (g, i) = Ig i | φ1 ∧ φ2 J K J K T (g1, i1) · (g2, i2) = Ig1 i1 •Ig2 i2 | φ1 ∨ φ2 J K J K J K | true t1 • t2 = ∀(g1, p = g2, π, φ, t) ∈ t1 | t ← t2 | false Ig · : ConAbs Interest → ConCore Tree (a) Common elements. J K Ig (∅, p, δ, λ) = [(g, p, false, D δ ∨ λ, [])] J K J K Preconditions ψ ::= CP (φ) Condition Precedent Ig (EL(φ), p, δ, λ) = [(g, p, φ, D δ ∨ λ, [])] | CS(φ) Condition Subsequent J K J K Ig (CS(φ), p, δ, λ) = [(g, p, φ ∧ (g re-enters), D δ ∨ λ, [])] | EL(φ) Executory Limitation J K J K Ig (CP (φ), p, δ, λ) = [(g, g, false, φ, []); | ∅ Unconditional J K (g, p, false, D δ ∨ λ, [])] J K Durations δ ::= Life(p) Life Estate Ig t1 :: t2 = T t1 @T t2 | Term(n) Term of Years J K J K J K | Abs Absolute Figure 2: Translating from ConAbs to ConCore

Limitation λ ::= φ Added Limitation prune E [i0; ... ; in] = [ik; ... ; in] Interest i ::= (ψ, p, δ, λ) where {k = max(j) | E ` πj } | i :: i c ::= (g, i) Conveyance 0 | c · c Sequencing lter E ((g, p, π, φ, is ) :: is) = i E ` φ then (lter E is) else (g, p, π, φ, is0 ) :: (lter E is) (b) ConAbs syntax. J K Interest tree t ::= (g, p, π, φ, t) · : E → [ ConCore interest ] | t :: t []J EK = [] J K (c) ConCore syntax. is E = ((prune E) ◦ (lter E)) is J K Figure 1: ConAbs and ConCore Syntax Figure 3: Semantics of ConCore

ough the ConAbs form is a simpler and more formal rep- the ConAbs versions to simpler predicates. Of these, the condi- resentation of legal conveyances, it is still a lile unwieldy for tion precedent case is interesting as it requires inserting a implicit computational analysis. For example, both natural duration and reversion for the grantor g before the main interest. limitations involve the same key concept: conditions under which Figure 3 precisely denes the behavior of interest trees with an interest expires. Figure 1c shows an even simpler representa- respect to a set of events E. E ` φ means the the formula tion for conveyances: an interest tree. An interest tree abstracts φ is true given the set of events E. e “prune” function nds out everything except the grantors and owners of interests and the latest interest in the tree whose pre-condition is true, and the condtions under which intersts terminates. A ConCore inter- removes all preceding interests. e “lter” function removes all est is composed of a grantor g, an owner p, a precondition π, a interests whose pre-conditions are true, recursively walking down postcondition φ and nally an interest sub-tree i. the interest tree (the is0 construct). In ConCore, a postcondition φ is a propositional logic formula J K over some set of events E, that when true, terminates the interest. Preconditions π are slightly more complicated: like postcon- 4. Implications and Future Work ditions, they are propositional formulae over events, but when the e previous section denes an abstract syntax for dening con- formula is true, all preceding interests are terminated. e sub-tree veyances and interests (ConAbs), converts the abstract represen- i is empty ([]) until the owner of the current interests conveys tation to a computationally tractable model (ConCore) and then away their interest. en it becomes the interest tree generated denes a straightforward semantics for how ConCore interest from that conveyance. trees are applied to sets of events E. Figure 2 denes a translation from ConAbs to ConCore. e is model captures several core features of property law: each D · function translates natural durations to the events that de- conveyance is a linear chain of interests, and composing con- ne them. eJ K function T · converts either a single conveyance, J K veyances results in a tree of interests. (e leaves of the tree still or a sequence of conveyances to a conveyance tree. e second form a linear chain of interests; the tree structure is there to en- T · case handles the formation of sub-trees as described above. sure that terminating a parent interest also necessarily termiantes I · J K is dened using g which handles most of the details. Essen- all the child interests created from it.) Given a set of events, the I · J K tially, g combines the durations and limitations into a single interest tree makes it easy to nd the current possessory interest: post-conditionJ K (D δ ∨ λ) and converts the pre-conditions from J K it is the rst element of the tree (the head), but if the interest has a sub-tree, then it is the head of that subtree. us there is one and only one possessory interest at any given time. Secondly, the semantics shown in Figure 3 precisely alter the interest tree in re- sponse to events. As events are added to the set E, the interest tree becomes a snapshot of possible interests aer the events in E have occurred. us the semantics support a “what-if” style reasoning: by changing the events in E, a user can interactively reason about the possible interests are some events have occured. is points to a possible direction of future work: by treating the interest trees as states, and individual events as transitions between states, we can build a state machine that describes all possible sequences of events, and possible interest structures. is resembles a computational model known as a Kripke structure that supports reasoning about event-based systems using a family of temporal logics. Adopting these logics for a ConCore-based model would allow us to express predicates such as “person P always eventually acquires possession”, and then automatically prove that such predicates are true for a given set of conveyances. A useful application of such a logic would be to express the Rule Against Perpetuities as a predicate that can be checked against a particular interest tree. In the case that a predicate is not true for a given tree, the proving mechanism could nd counter-examples that invalidate the predicate. For the Rule Against Perpetuities, the prover would nd interests that are rendered invalid by the Rule, together with possible sequences of events that cause those interests to vest too late. We have implemented the translation from ConAbs to ConCore, a version of the ConCore semantics as well as parser from a human-readable language resembling natural conveyances to ConAbs. is allows users to write lists of conveyances in human- readable form, and generate a visual representation of an interest tree. Our implementation also provides a interface to specify events that have occured, and see the interest tree being modied according to the ConCore semantics. e entire system is acces- sible via a web interface on modern browsers and has been tested with a set of examples based on popular casebooks.